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OUAIN'S 


ELEMENTS    OF    ANATOMY 


EDITED    BY 


EDWARD    ALBERT    SCHAFER,  LL.D.,    F.R.S. 

PBOKh:SSOR     OF     PUVSIOLOGV    AND     HISTOLOOV     IX     USIVEU31TY    COLt.EOE,     LON'DOK, 


GEORGE  DANCER  THANE, 

PROFESSOR     OF     AXATOMV     IX      UXIVEP^ITV     COLLEGE,      LOXDOX. 


IN   THREE   VOLUMES. 

.         .     VOL.     I  —  PART    L 

EMBRYOLOGY. 
By    PEOFESSOE    SCHAFEE. 

illustrated  by  200  engravings,   many  of  which   are  coloured. 


l^tiu    en  it  I  on. 


LONGMAXS,     GREEN,     AND     CO. 

39    PATEEXO.STER    ROW,     LONDON 
NEW    YORK    AND   BOMBAY 

1898 
[AU  rights  rescrixd.] 


BIBLIOGRAPHICAL    NOTE. 

Ninth  Edition,  2  Fo/s.,    8to.   November,    1882;     FoL    7.    Reprinted    Mm-cli,  1884; 
Odoher,  1887.     FoZ.  7/.  Reprinted  Decemler,  1883 ;  ^pn7,  1887. 

Teni/i  Edition,  edited  hy  E.  A.  ScJidfer  and  G.  D.  Thane,  in  8  separately  issued 

Parts  and  an  Appendix,  1890-6. 
Vol.  I.,  Parti.  {Embryology),  first  sepjarate  issue,  October,  1890;  Reprinted,  October, 

1892  ;  also,  tvith  additions  and  emendations,  January,  1896,  and  again,   with 

further  additions,  November,  1898. 


CONTENTS    OF    PART    I. 


Intuoductiox  : 
General  CoNsiDEnAiioxs 
Plan  of  Oiganisalion 
The  Vertebrate  typo    . 


PAGE 


Segmentation  of  the  Body 
Homology  . 
Symmetry  of  Form. 
Descriptive  terms 


PACK 

2 

4 

.       4 

.       4 


EMBRYOLOGY. 


6,  9 

9 
II 

12 

12 

14 
14 


ft 


Btkucttke  of  Ovakian  Ovvm  ;  Matuka 
TioN  OF  Ovum    .... 
Formation  of  Polar  Globules 

Fertilization 

Cleaning  of  Polar  Globules. 
Theory  of  Minot     .         .         .         • 
Theory  of  AVeissmann 
Recent  Literature  of  the  Ovum 
Segmentatiox  of  the   Ovvm  ;    Forma- 
tion OF  THE  Blastoderm    .        .    i6,  17 
Gastrula    Condition    of   the    Vertebrate 

Ovum •     •     ^^ 

Views  concerning   the    Gastrulation  of 

A^'ertebrates 22 

Inversion  of  Blastodemiic.Laj-ersin  some 

Mammals  .  .  .  •  •  •  "~ 
Historical  view-of  Blastoderm  .  .  . 
Characters  of  the  Blastodermic  Layers  . 
Parablast  theory  of  His  .... 
Mesenchyme  theory  of  Hertwig  ,  .  . 
Kecent  Literature  of  Blastoderm  . 
Early  Changes  in  Blastoderm         .     . 

Neural  Canal 

Notochord         ...... 

Separation  of  the  Embryo 

Cleavage  of  Mesoblast        .         .         .     - 

Formation  of  Body  Cavity 

Formation  of  Mesoblastic  Somites      .     . 

Cerebral  Vesicles 

Heart  and  Vascular  System        .         .     . 

Kecent  Literature  of   Early  Changes  in 

Blastoderm  .         .         .         .         • 

Development    of    the     Fcetal    ]\Iem- 

ERANES  ;  Attachment  of  Ovl'm  to 

Uterl's 

Formation  of  the  Amnion  and  Chorion   . 

Formation  of  the  Allantois     . 

Changes   in   the   Uterus    and    mode    of 

attachment  of  Ovum  to  Uterus    . 
The  Placenta     . 

Separation  of  the  Decidua  at  birth,  and 
regeneration  of   the   Uterine  Mucous 
Membrane  ...... 

Keceut  Literature  of  the  Decidua        ,     . 

Development  of  the  XERVors   System 

Of  the  SpinalCord         .... 

Of  the  Brain 


23 
24 

25 
26 
27 
30 
30 
32 
34 
36 
36 
36 
37 


41 


42 
42 
43 

46 
53 


55 
55 
57 
57 
61 


Further  details  regarding  the  develop- 
ment of  special  parts  of  the  Brain 
The  fifth  cerebral  vesicle  :  Bulbar  vesicle 

or  Metencephalon 

The   fourth  cerebral  vesicle  :  Cerebellar 
vesicle  or  Epencephalon.         .         .     • 
The    third    cerebral    vesicle  :     Mesen- 
cephalon :  Mid-brain .... 

The  second  cerebral  vesicle  :  Thalamen- 
cephalon         ....•• 

The  fii-st  cerebral  vesicle  :  Prosencephalon 
The  Olfactory  Lobes  .         .         .        •     . 
Formation  of  the  FissuresandConvolutions 
Development  of  the  Nerves 

Spinal  nerves     .         .         ...         •     . 

Cranial  nerves         ..... 

Optic  nerves      ...... 

Olfactory  lobe 
Sympathetic  nerves  and  ganglia         .     . 
Kecent  Literature  of  the  development  of 
the  Nervous  System    . 
Development  of  the  Eye 
Of  the  Ketina 
Of  the  Lens 
Capsule  of  the  Lens 
Vitreous  humour 
Corneal  epithelium 
Sclerotic    .... 

Choroid  coat  . 
Accessor}"  structures  . 
LachrjTnal  gland  . 
Lachrymal  canals  and  ducts  .  .  . 
Development  of  the  Ear  ;  The  Laby- 
rinth          

Accessory  parts  of  the  Organ  of  Hearing 

External  and  Middle  Ear    . 

Develop.ment  of  the  Nose       .        .     • 

Kecent  Literature  of  the  development  of 

the  Sense  Organs         .... 

Development  of  the  Ali.mentary  Canal 

Of  the  Mouth  and  parts  in  connection 

■with  it 

Pharynx 

Tongue      

(Esophagus,  Stomach,  and  Intestines 

The  Mesentery 

The  Spleen 


63 
63 

:f 

68  ^ 
71  -7 
71  ^  I 

'^ 

79 

79  . 
Si  ^ 

Si 

S3 
86 
86 
S7 
S7 
87 
88 
S8 
89 
89 
89 

S9 

93 
95 

98 
99 

99 

lOI 

102 

103 
104 
108 


S:v 


IV 


CONTENTS    OF    PAET   I. 


Formation  of  the  Anus       .         .         . 

FOKMATION  OF  THE   GlANDS   OF  THE   AlI 
MENTAKY  CaNAL  ... 

The  Lungs  ,         .         .         .         . 

The  Trachea  and  Larynx 

The  Thyroid  Body     .... 

The  Thymus  .         . 

The  Liver 

The  Pancreas         ..... 

Eecent  Literature  of  the  development  of 
the  Alimentary  Canal  and  Glands  . 
Development    of    the    Urixaey    and 
Genekative  Oegans 

The  Wolffian  duct  and  hody 

Supra-renal  Capsules 

The  permanent  Kidnej^s     , 

The  Urinary  Bladder 

The  Miillerian  duct    . 

The  Germinal  Epithelium 

Development  of  the  Ovary. 

Of  the  Testicles     , 

Descent  of  the  Testicles     . 

The  External  Organs 

Table  of  Generative  Organs 

Recent  Literature  of  the  development  of 
the  Urinary  and  Generative  Organs 
Formation  of  the  Vascular  System 

Development  of  the  Heart 

Peculiarities  of  the  Foetal  Heart 

Development  of  the  principal  Arteries 


page 
1 08 


109 
109 
no 
no 
III 
112 
"3 

113 

115 
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122 
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124 
124 

125 

126 
127 
130 

132 
134 
134 
146 
146 


PAGK 


Destination  of  tlie  fourth  and  fifth  Arte- 
rial Arches 

Development  of  the  principal  Veins. 

Peculiarities  of  the  foetal  Organs  of  Circu 
lation 

The  Foramen  Ovale   . 

The  Eustachian  Valve    . 

The  Ductus  Arteriosus 

The  Umbilical  Vessels    . 

Course  of  the  blood  in  the  Foetus 

Changes  in  the  Circulation  at  Birth 

The  Lymphatic  System 

Eecent  Literature  of  the  development  of 
the  Vascular  System  . 
Deat:lopment  of  the  Serous  Cavities 
ANT)  OF  the  Muscles  and  Skeleton 

The  Serous  Cavities 

Development  of  the  Muscles 

Formation  of  the  head  Muscles  and  evi 
deuces  of  head  segmentation  . 

Development  of  the  Vertebral  Column 

Ribs  and  Sternum      .... 

The  Limbs     ..... 

The  Cranium     ..... 

Formation  of  the  visceral  skeleton  of  the 
Head  ;  cartilaginous  bars  of  the  visceral 
arches         ...... 

Formation  of  the  Auditory  Ossicles    .     . 

Recent  Literature  ofthe  development  of  the 
Serous  Cavities,  Muscles,  and  Skeleton  169 


150 
151 

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155 
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156 

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156 
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157 

158 

159 
159 
159 

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163 
163 
165 


166 
167 


ELEMENTS    OF    ANATOMY. 


INTPtODUCTION. 


Anatomy,  in  its  most  extended  seuse,  is  the  science  which  deals  with  the 
structure  of  organized  bodies.  It  is  divided  into  departments  according  to  its 
subjects  ;  such  as  Human  Anatomy  ;  Comparative  Anatomy,  or  the  study  of  the 
structure  of  different  animals  ;  and  Vegetable  Anatomy,  comprehending  the 
structure  of  plants. 

On  examining  the  structure  of  an  organized  body,  we  lind  that  it  is  made  up  of 
members  or  organs,  by  means  of  which  its  functions  are  executed,  such  as  the  root 
stem  and  leaves  of  a  plant,  and  the  heart,  brain,  stomach  and  limbs  of  an  animal  ; 
and  ftirther,  that  these  organs  are  themselves  made  up  of  certain  constituent 
materials  named  tissues  or  textures,  such  as  the  cellular,  woody,  and  vascular  tissues 
of  the  vegetable,  or  the  osseous,  muscular,  connective,  vascular,  nervous,  and  other 
tissues,  which  form  the  animal  organs. 

Most  of  the  tissues  occur  in  more  than  one  organ,  and  some  of  them  indeed,  as 
the  connective  and  vascular,  in  nearly  all,  so  that  a  multitude  of  organs,  and  these 
greatly  diversified,  are  constructed  out  of  a  small  number  of  constituent  tissues  ; 
and  pai'ts  of  the  body,  differing  widely  in  form,  construction,  and  uses,  may  agi-ee  in 
the  nature  of  their  component  materials.  Again,  as  the  same  tissue  possesses  the 
-same  essential  characters  in  whatever  organ  or  region  it  is  found,  it  is  obvious  that 
the  structure  and  properties  of  each  tissue  may  be  made  the  subject  of  investigation 
apart  from  the  organs  into  whose  formation  it  enters. 

The  foregoing  considerations  have  led  to  the  subdivision  of  anatomy  into  two 
branches,  the  one  of  which,  under  the  name  "  General  Anatomy,"  or  "  Histology," 
treats  of  the  minute  structure  of  the  component  tissues  of  the  body  ;  the  other, 
named  "  Special  or  Descriptive  Anatomy,"  treats  of  its  several  organs,  members,  and 
regions,  describing  the  outward  form  and  internal  structure  of  the  parts,  their 
relative  situation  and  mutual  connection,  and  the  successive  conditions  which  they 
present  in  the  progress  of  their  formation  or  development. 

To  the  description  of  the  origin  and  formation  of  organs  in  the  embryo,  a  special 
chapter  is  devoted  in  this  work,  under  the  name  Embryology. 

The  study  of  anatomy  may  be  viewed  in  two  different  aspects  ;  viz.,  the  physio- 
logical and  the  morphological.  In  the  former,  anatomy  supplies  the  materials 
relating  to  structure  from  which  an  explanation  is  sought  of  the  uses  or  functions 
of  organs  by  the  physiologist ;  and  for  this  purpose  the  study  of  histology  is  of 
particular  service.  In  its  morphological  aspect,  anatomy  investigates  and  combines 
the  facts  relating  to  the  structure  and  relations  of  organs,  from  which  may  be 
deduced  general  principles  as  to  the  construction  of  the  human  body  or  that  of 

VOL,  I.  B 


2  INTRODUCTION. 

animals.  In  the  determination  of  these  general  principles,  or  laws  of  morphology ;- 
it  is  necessary  to  combine  the  knowledge  of  the  anatomy  and  development  of 
animals  with  that  of  man. 

PLAN    OF    ORGANIZATION. 

Vertebrate  type. — The  general  plan  of  construction  of  the  human  body  agrees 
closely  with  that  which  prevails  in  a  certain  number  of  animals,  viz.,  mammals, 
birds,  reptiles,  amphibia,  and  fishes,  and  is  known  as  the  vertebrate  type  of  organi- 
zation. The  main  featm-e  of  that  type,  and  that  from  which  its  name  is  derived,, 
belongs  to  the  internal  skeleton,  and  consists  in  the  existence  of  a  median  longi- 
tudinal column,  which  extends  through  the  whole  trunk,  and  is  composed  in  the  fully 
developed  state  of  a  series  of  bones  termed  vertelirce.  This  vertebral  column  is  formed 
in  the  early  embryo  around  a  simple  rod-like  structure,  the  primitive  skeletal  axis, 
which  is  called  the  notochord,  and  which  in  most  vertebrate  animals  disappears  to  a 
greater  or  less  extent  in  the  course  of  development.  The  more  solid  portions  of  the 
vertebra  immediately  surrounding  the  notochord  are  known  as  the  todies  or  centra 
(figs.  2  and  3),  and  constitute  a  pillar  around  which  the  other  parts  are  grouped 
with  a  certain  regularity  of  structm-e.  At  one  extremity  of  this  pillar  is  situated  the 
head,  snowing  in  almost  all  the  animals  formed  upon  this  type  a  greater  development  of 
its  constituent  parts  ;  and  at  the  other  the  tail  in  which  an  opposite  character  or  that 
of  diminution  prevails  ;  while  on  the  sides  of  the  main  part  or  trunic,  there  project, 
in  relation  with  some  of  the  vertebral  elements,  two  pairs  of  symmetrical  limis. 

The  head  and  trunk  contain  the  organs  or  viscera  most  important  to  life,  such  as- 
the  alimentary  canal  and  the  great  central  organs  of  the  vascular  and  nervous 
systems,  while  the  limbs,  from  which  such  principal  organs  are  absent,  are  very 
variable  and  diifer  widely  in  the  degree  of  their  development  among  the  various 
animals  formed  upon  the  vertebrate  type.  In  man  and  the  higher  animals  the  trunk 
is  divisible  into  neck,  chest,  abdomen,  and  pelvis. 

The  vertebrate  form  of  skeleton  is  invariably  accompanied  by  a  determinate  and 
conformable  disposition  of  the  other  most  important  organs  of  the  body,  viz.  : — 
firstly,  the  existence  on  the  dorsal  aspect  of  the  vertebral  axis  of  an  elongated  cavity 
or  canal  which  contains  the  brain  and  spinal  cord,  or  central  organs  of  the  nervous 
system  ;  and  secondly,  the  existence  on  the  ventral  aspect  of  the  vertebral  axis  of  a 
larger  cavity,  the  visceral  cavity,  body  cavity  or  ccelom,  in  which  are  contained  the 
principal  viscera  connected  with  nutrition  and  reproduction,  such  as  the  alimentary 
canal,  the  heart  and  lungs,  the  great  blood-vessels,  and  the  urinary  and  generative 
organs. 

The  general  disposition  of  the  parts  of  the  body  and  of  the  more  important 
viscera  in  their  relation  to  the  vertebral  axis  are  shown  in  the  accompanying 
diagrams  of  the  external  form  and  longitudinal  and  transverse  sections  of  the  human 
embryo  at  an  early  period  of  its  existence. 

Segmentation  of  the  body. — The  vertebrate  type  of  organisation  in  the  repetition  of 
similar  structural  elements  iu  a  longitadinal  series,  has  a  segmented  character,  especially  in 
the  axial  portion  of  the  body,  and  this  segmentation  affects  more  or  less,  not  merely  the 
skeletal  parts  of  its  structure,  but  also,  to  some  extent,  its  other  component  organs. 

A  segmented  plan  of  construction  is  by  no  means  restricted  to  vertebrate  animals,  but  exists 
in  several  other  classes  of  the  animal  kingdom,  as  is  most  conspicuously  seen  in  the  Arthropoda, 
such  as  insects  and  Crustacea,  and  in  the  Annelida  or  worms.  These  animals,  however,  although 
showing  a  serial  repetition  of  parts  of  like  structure,  are  not  considered  to  belong  to  the  verte- 
brate type  of  organi2;ation. 

In  the  human  embryo,  as  in  that  of  aU  vertebrate  animals,  the  segmentation  is  most  marked 
in  the  muscular  system,  the  nervous  and  osseous  systems  becoming  for  the  'most  part  corres- 
pondingly marked  off  :  in  the  adult  the  osseous  and  nervous  systems  retain  in  great  measuie 
the  segmentation  which  has  thus  been  produced,  although  in  the  muscular  system  it  has 


PLAN    UF    OUGAiNlZATIoX, 


^^S-  1- — Diagram  of  an  early  hciian  embryo.     (Allen  Thomson.) 
s,  s,  indications  of  the  vertebral  divisions  along  the  line  of  the  back  ;  r,  m,  upper  limb  ;  t,  f,  lower 
limb  ;  u,  umbilical  cord      In  the  cranial  part  the  divisions  of  the  brain  are  indicated,  together  with 
tne  eye,  and  au,  the  auditory  vesicle  ;  near  b,  the  visceral  arches  and  clefts  of  the  head,  forming  /h/c/- 
alia  the  rudiments  of  the  upper  and  lower  jaws. 

Fig,  2.— Semi-diagramjiatic    view   of  a    longitudinal   section   of    the   embryo   represented    in 

FIGURE    1;     SHOWING    THE    RELATIONS    OF    THE    PRINCIPAL    SYSTEMS   AND    ORGANS.       (Allen  ThomSOn. ) 

1,  2  3,  4,  5  primary  divisions  of  the  brain  in  the  cranial  pai-t  of  the  neural  canal  :  n,  n,  spinal 
cord  in  the  vertebral  part  of  the  canal ;  s,  spinous  process  of  one  of  the  vertebra  ;  ch,  chorda  dorealis  run- 
ning through  the  axis  ol  the  vertebral  centra  :  ch',  the  same  extending  into  the  base  of  the  cranium  •  a 
doreal  aorta  ;  p,  pharyngeal  cavity  ;  /,  i,  alimeufcvry  canal  :  h,  ventricular  part  of  the  heart,  ^nth 
which  the  arterial  bulb  is  seen  jouiiug  the  aorta  by  arches  ;  b,  visceral  arches  of  head  ;  I,  Hver  •  v 
woltlian  body  ;  v,  urinary  vesicle  or  allantois,  joining  the  intestine  in  the  cloaca,  cl ;  u,  u\  umbilicus. ' 

Fig.  3.— Transverse  section  (diagrammatic)  of  the   trunk  of  the  embryo  through  the  upper 

LIMBS.     (Allen  Thomson.) 
m   spinal  cord  ;  n,  neural  or  dorsal  arch,  including  bone,  muscle,  skin,  roots  of  the  nerves,  kc.  : 
cA    chorda  dorsahs,  surrounded  by  the  vertebral  body  or  centrum  ;  v,  ventral  or  visceral  arch,  or  waU 
01  tne  body  ;  p,  p,  body  cavity  ;  i,  alimentary  canal ;  h,  heart  ;  7,  I,  the  rudimentary  Hmbs. 

Fig.  i.— First  dorsal  vertebra  with  the  first  rib  and  upper  part  of  the  sterkuji,  seen  from 

ABOVE.       |. 

C,  centi-um  ;  N,  neural  cavity  ;   F,  cavity  of  the  chest,  visceral  cavity. 

B  2 


4  INTRODUCTION. 

become  greatly  obscured.  To  the  original  segments  in  the  embryo  the  terms  protovcvtebrcs, 
mesoMastic  somites  or  myotomes  have  been  applied ;  those  segments  or  metameres  which  are 
traceable  in  the  adult  are  often  spoken  of  as  vertelral  segments.  In  the  limbs,  although  there 
is  strong  reason  for  believing  that  they  have  originated  as  outgrowths  of  certain  segments  of 
the  trunk,  the  repetition  of  such  vertebral  elements,  and  their  primitive  connection,  are 
greatly  obscured. 

Homology. — A  certain  agreement  in  structure,  situation  and  connection  of 
parts  or  organs  constitutes  what  is  called  homology,  and  this  term  is  generally 
employed  to  indicate  the  morphological  identity  of  representative  parts  in  different 
animals,  which  may  be  considered  to  have  its  cause  in  community  of  origin  {homo- 
geny,  Lankester),  while  the  anatomical  correspondence  of  parts  which  are  repeated 
in  the  same  animal  may  be  more  exactly  distinguished  as  serial  homology  {homo- 
dynamy,  Gegenbaur).  Thus  the  arm-bone  or  humerus  of  a  man  is  homologous 
(homogenetic)  with  the  upper  bone  of  the  fore  limb  of  a  quadruped,  or  of  the  wing 
of  a  bird,  while  it  is  at  the  same  time  serially  homologous  (hemodynamic)  with  the 
thigh  bone  of  man  himself,  or  any  other  vertebrate  animal.  It  has  farther  been 
found  convenient  to  express  by  the  word  analogy  that  kind  of  resemblance  among 
the  organs  of  animals  which  depends  upon  similarity  of  function,  and  although  it 
may  be  accompanied  by  considerable  agreement  in  structure,  yet  is  not  rendered 
complete  by  anatomical  relation  and  connection  :  for  example,  the  gills  of  a  fish,  of 
a  crab,  and  of  a  mussel,  serving  the  same  function,  are  analogous  organs,  but  in  no 
sense  homologous,  as  all  morphological  correspondence,  or  genetic  relation,  is  wanting 
between  them.  Thus  also,  the  upper  limb  of  a  man,  the  fore  limb  of  a  quadruped, 
the  wing  of  a  bird,  and  the  pectoral  fin  of  a  fish  are  homologous  but  not  analogous 
structures,  the  wing  of  a  bat  and  the  wing  of  a  bird  are  both  homologous  and 
analogous,  while  the  last  is  analogous  to  but  not  homologous  with  the  wing  of  an 
insect. 

Symmetry  of  form. — A  remarkable  regularity  of  form  pervades  the  organi- 
zation of  certain  parts  of  the  body,  especially  the  whole  of  the  limbs,  the  head  and 
neck,  and  the  framework,  at  least,  and  external  walls  of  the  trunk  of  the  body. 
Thus,  if  we  conceive  the  body  to  be  divided  equally  by  a  plane  which  passes  from  its 
dorsal  to  its  ventral  aspect  {median  pkine),  the  two  halves,  in  so  far  as  regards  the 
parts  previously  mentioned,  correspond  almost  exactly  with  each  other,  excepting  by 
their  lateral  transposition, — and  the  human  body  thus  shows  in  a  marked  manner 
the  character  of  lilateral  symmetry.  There  is,  however,  a  departure  from  this 
symmetrical  form  in  the  developed  condition  of  certain  of  the  internal  organs,  such 
as  the  alimentary  canal  from  the  stomach  downwards,  the  heart  and  first  part  of  the 
great  blood-vessels,  the  liver,  spleen,  and  some  other  viscera. 

Descriptive  terms. — In  the  description  of  parts  so  numerous,  so  various  in 
form,  and  so  complex  in  their  connections  as  those  composing  the  human  body,  there 
is  diJB&culty  in  finding  terms  which  shall  indicate  with  sufficient  precision  their 
actual  position  and  their  relation  to  the  rest  of  the  organism.  This  difficulty  is 
farther  increased  by  the  exceptional  erect  attitude  in  which  the  trunk  of  the  human 
body  is  placed  as  compared  with  the  horizontal  position  in  animals.  Hence,  a 
number  of  terms  have  long  been  in  use  in  human  anatomy  which  are  understood  in 
a  technical  or  restricted  sense.  For  example,  the  median  plane,  already  referred  to, 
being  that  by  which  the  body  might  be  divided  into  right  and  left  lateral  halves, 
and  the  middle  or  median  line  being  that  in  which  the  median  plane  meets  the  surface 
of  the  body,  the  words  internal  and  external  are  used  to  denote  relative  nearness  to 
and  distance  from  this  plane  on  either  side,  and  may  be  replaced  by  mesial  and 
lateral.  The  terms  sagittal,  frontal,  and  coronal,  are  also  used  in  indication  of 
direction  within  the  body  :  sagittal  denoting  a  dorso-ventral  direction  in  or  parallel 
to  the  median  plane,  frontal  or  coronal  a  transverse  direction  perpendicular  to  that 


DESCKIPTIVE    TERMS. 


plane.  The  words  anterior  and  /mterior,  superior  and  vtfenor,  and  several  others 
iudicatiiig  position,  are  employed  in  human  anatomy  strictly  with  reference  to  the 
erect  posture  of  the  body.  But  now  that  the  more  extended  study  of  comparative 
anatomy  and  embryonic  development  is  largely  applied  to  the  elucidation  of  the 
human  structure,  it  is  very  desirable  that  descriptive  terms  should  be  sought  which 
may  without  ambiguity  indicate  position  and  relation  in  the  organism  at  once  in 
man  and  animals.  Such  terms  as  dorsal  and  ventral,  neural  and  visceral,  cephalic 
and  caudal,  central  and  peripheral,  proximal  and  distal,  axial  and  aprpendicxdar ,  pre- 
axial  and  postaxial,  are  of  this  kind,  and  ought,  whenever  this  may  be  done  con- 
sistently with  sufficient  clearness  of  description,  to  take  the  place  of  those  which  are 
only  applicable  to  the  peculiar  attitude  of  the  human  body,  so  as  to  brin"-  the 
language  of  human  and  comparative  anatomy  as  much  as  possible  into  conformity. 
In  many  instances,  also,  precision  may  be  obtained  by  reference  to  certain  fixed 
relations  of  parts,  such  as  the  vertebral  and  sternal  aspects,  the  radial  and  ulnar,  an^ 
the  tibial  aud  fibular  borders,  the  fiexor  and  cilcn.^or  surfaces  of  the  limbs,  and 
similarly  in  other  parts  of  the  body. 


EMBRYOLOGY/ 

By  E.  A.  SCHAFER. 


GENERAL   DEVELOPMENT. 

FORMATION"    OF    THE    BLASTODERM. 

STRUCTURE     OF     THE     OVUM    AND     CHANGES    PRIOR     TO     SEGMENTATION. 

The  human  body  with  all  its  tissues  and  organs  is  the  product  of  the  development 
of  a  single  nucleated  cell,  the  egg-cell,  germ-cell,  or  ovum,  which  is  formed  within 
the  principal  reproductive  organ  of  the  female  or  ovary.  The  commencement  of 
development  is  preceded  by  certain  changes  in  the  ovum,  which  usually  occur  soon 
after  its  discbarge  from  the  ovary,  and  consist  (1)  in  the  emission  of  certain 
constituents  of  the  nucleus  which  form  the  so-called  polar  globules  ;  (2)  in  the 
accession  of  the  nucleus  of  a  sperm-cell  or  spermatozoon,  which  is  formed  within  the 
reproductive  organ  of  the  male  (testicle),  and  which,  blending  with  the  remaining 
part  of  the  nucleus  of  the  ovum,  appears  to  take  the  place  of  the  part  which  was 
discharged  in  the  form  of  the  polar  globules. 

An  account  of  the  structure  of  the  ovum,  and  of  the  manner  in  which  the  above 
changes  are  effected,  may  therefore  appropriately  precede  the  description  of  the 
actual  course  of  development  of  the  ovum. 

Structure  of  the  ovarian  ovum. — The  human  ovum  resembles  that  of  all  other 
mammals  (with  the  exception  of  monotremes)  in  its  minute  size.  Immediately 
before  the  time  of  its  discharge  from  the  Graafian  foUicle  of  the  ovary  in  which  it 
has  been  formed,  it  is  a  small  spherical  vesicle  measuring  about  yig-th  inch  {'2  mm.) 
in  diameter,  and  is  just  visible  as  a  clear  speck  to  the  naked  eye.  When  it  is 
examined  with  the  microscope,  it  is  found  to  be  invested  by  a  comparatively  thick, 
clear  covering.  This,  when  the  centre  of  the  ovum  is  exactly  focussed,  has  the 
appearance  in  optical  section  of  a  clear  girdle  or  zone  encircling  the  ovum  (fig.  5), 
and  was  hence  named  zona  pellucida  by  von  Baer  (1827).  But  on  more  careful 
examination  with  higher  magnifying  powers,  and  especially  by  the  examination  of 
sections,  there  is  not  much  difficulty  in  making  out  the  existence  of  strise  passing 
radially  through  the  membrane  (fig.  6,  z^f).  On  this  account,  and  especially  since  a 
similar  radially  striated  membrane  forms  a  characteristic  part  of  the  investment  of 
the  ovum  in  many  animals  belonging  to  widely  different  classes,  it  is  more  convenient, 
in  place  of  the  name  zona  pellucida,  which  has  been  exclusively  used  to  designate 
this  investment  in  mammals,  to  employ  the  more  general  term  zona  radiata,  or  to 
gpeak  of  it  simply  as  the  striated  membrane  of  the  ovum.  " 

The  zona  radiata  of  the  mammalian  ovum  is  sufficiently  tough  to  prevent  the 
escape  of  the  contents  of  the  ovum,  even  when  subjected  to  a  considerable  amount 
of  pressure.     If,  however,  the  pressure  be  excessive,  the  tunic  splits,  and  the  soft 

'  It  is  mainly  owing  to  the  researches  of  His,  published  principally  in  the  important  monograph 
"  Anatomic  nienschlicher  Embryonen  "  (Leipzig,  1880-1885),  that  our  knowledge  of  the  development  of 
Ihe  human  embryo  is  now  far  more  complete  than  was  the  case  when  the  last  edition  of  this  work  was 
andertaken,  and  we  are  therefore  able  to  keep  more  closely  than  was  before  possible  to  the  human 
species  in  following  the  course  of  derelopment  of  the  ovum.  For  the  elucidation,  however,  of  many  of 
the  details  of  development,  especially  in  its  earlier  stages,  it  will  still  be  necessary  to  refer  continually 
to  facts  which  have  been  made  out  only  from  the  study  of  the  embryology  of  other  mammals,  as  well  as 
birds,  reptiles,  fishes,  and  even  invertebrata. 


STiturnuH  (»r  thk  ovim.  7 

contents  are  extruded  (Hir.  "i,  b).  The  striiK  in  tbu  membrane  are  believed  to 
be  minute  purcs,  and  aie  supposed,  while  the  ovum  is  yet  within  the  (Jraafian 
tblhcle,  to  permit  tlie  passjige  of  grannies  of  nutrient  material  into  the  interi<jr  of 
the  ovum.  After  the  ovum  is  disehar'^ed  from  the  follicle,  the  spermatozoa  may 
perhaps  find  their  way  into  the  ovum  throuj^h  these  pores.  AccoTdlng  to  Ketzius 
the  protoi)lasm  of  the  ovum  is  united  with  the  fullicle-cells  by  fibres  which  pats 
through  the  pores  of  tiie  zona. 

Inimedialoly  surroundinj,'-  tlie  zona  ladiata.  as  the  ovum  lies  within  the  mature  Graafiiiii 
follicle,  is  a  thin  stratum  of  ■,'ianular  substJince.  probably  deposited  upon  the  exterior  of  the 
ovum  by  the  iuneiouost  cells  of  the  discus  prolijrerus.  which  immediately  encircle  the  ovum 
within  the  follicle.  When  the  Graafian  follicle  bursts  and  the  ovum  is  set  free,  this  ;;ranular 
material  ajipears  to  imbil)e  water,  and.  as  is  .-rpecially  noticeable  in  the  ovum  of  the  rabbit, 
swells  uji  into  a  clear  ^'elatinous  envelope,  which  has  been  termed,  from  a  possible  homolo-ry 
with  the  white  of  the  bird's  ej-'t-'.  the  till/m/irn.  But  in  the  mammal  this  structure  ha.s  not 
the  nutritive  importance  to  the  embryo  which  is  possesseil  by  the  corresponding  fonnation 
in  the  bird,  and  it  disiippears  during  the  pa-ssage  of  the  ovum  down  the  Fallopian  tulje. 

The  substance  of  the  ovum  within  the  tunica  radiata  is  known  as  the  vitellus 
or  ijoJk  (fig.  G,  vi).  It  is  a  soft  semi-fluid  substance,  composed  mainly  of  proto- 
plasm, winch    is    filled  with  globides    and   gninules   (yolk-granules)   of  different 


Fig.  T>. — Ovarian  ovum  of  a  mammifek.     (Alien  Thomson. ) 

a,  the  entire  ovum,  viewed  under  pressure  ;  the  granular  cells  have  been  removed  from  the  outer 
•surface,  the  germinal  vesicle  is  seen  in  the  yolk  substance  within  ;  h,  the  external  coat  or  zona  burst  by 
increasetl  pressiue,  the  yolk  protoplasm  and  the  genninal  vesicle  having  escaped  from  within  ;  c,  germi- 
nal vesicle  more  freed  from  the  volk  substance.     In  all  of  them  the  macula  is  seen. 


Fig.  6. — Ovum  of  the  cat  ;  highly  magnified,     semi-diagrammatic.     (E.  A.  S.) 

zp,  zona  pelliicida,  showing  mdiated  structure ;   ci,  vitellus,  round  which  a  delicate  membrane  is 
seen  :  ;/'•,  germinal  vesicle ;  fjs,  germinal  spot. 

sizes,  but  all  small,  and  possessing  a  high  index  of  refraction.  Examined  in  the 
fresh  condition,  the  protoplasm  between  the  granules  looks  perfectly  clear  and 
structureless,  but  after  treatment  with  suitable  reagents,  it  may  be  seen  to  consist  of 
a  fine  reticulum,  which  is  especially  fine  and  close  near  the  periphery  of  the  ovum, 
and  also  aroimd  the  germinal  vesicle,  at  which  places  the  yolk  granules  are  in  less 
amount  than  elsewhere.  The  substances  which  occur  ^vithin  an  ovum  other  than 
the  nucleus  and  protoplasm,  may,  as  in  cells  generally,  be  collectively  designated 
^'  deutoplasm  "  ;  they  are  regarded  as  funiishiug  a  supply  of  nutrient  matter  to  the 
protopla.sm  dm-ing  the  earlier  stages  of  development. 

Embedded  in  the  protoplasmic  vitellus,  usually  eccentrically,  is  a  large  spherical 


8  STRUCTURE    OF    THE    OVARIAN    OVUM. 

nucleus,  which  was  termed  by  its  discoverer,  Purkinje,  the  gmninal  vesicle.^  This^ 
which  is  about  -g^o^h  inch  in  diameter,  has  all  the  characters  of  the  nucleus  of  a 
cell.  It  consists  of  a  nuclear  membrane  enclosing  a  clear  material  or  matrix, 
embedded  within  which  may  be  seen  strands  of  karyoplasm,  enclosing  one  or  more 
well-marked  nucleoli  (fig.  6,  gv).  Frequently  there  is  but  one  nucleolus,  which  is 
then  large  and  prominent,  and  has  received  the  name  of  germinal  spot  {macula  ger- 
ininaiiva,  Wagner,  1805). 

There  is  some  doubt  whether,  before  fertilization,  there  is  another  membrane  (vitelline 
membrane)  enclosing-  the  vitellns  within  the  zona  radiata.  The  evidence  of  the  presence  of 
i>uch  a  membrane  is  by  no  means  clear,  although  its  existence  has  been  maintained  by  very 
competent  observers  (v.  Beneden,  Balfour). 

The  mammalian  ovum  (that  of  monotremes  alone  excepted)  differs  from  that  of  other 
vertebrates  in  the  relatively  small  amount  of  nutritive  material  (yolk  granules,  deutoplasm) 
which  is  embedded  in  its  protoplasm.  In  fishes,  amphibia,  reptiles,  and  especially  in  birds, 
the  amount  of  such  nutritive  material  is  vastly  g'reater  than  that  of  the  protoplasm  itself,  so 
that  the  very  existence  of  the  latter  is  obscured  in  most  parts  of  the  ovum,  and  it  is  only  in 
the  immediate  neighbourhood  of  the  germinal  vesicle  that  the  protoplasm  can  be  distinctly 


Fig.  7. — Diagram  of  a  holoblastic  (alecithal)  ovum  (A)  and  of  a  mekoblastic  (telolecithal} 

OVUM  (B).     (E.  A.  S. ) 

Only  a  small  part  of  the  latter  is  represented.  The  yolk  or  food  material  is  represented  in  both  hj 
clear  globules,  which  in  B  are  seen  vastly  to  preponderate,  except  in  the  immediate  neighbourhood  of 
the  germinal  vesicle. 

recognized  (fig.  7,  B).  It  is  here  also  that,  after  fertilization,  the  more  active  changes  in  the- 
ovum  occur,  and  it  is  this  part  alone  in  which  in  the  bird  and  most  other  oviparous  vertebrate? 
the  process  of  division  or  segmentation  of  the  yolk  and  consequent  formation  of  embryonic  cells- 
proceeds.  Hence  these  ova  are  said  to  undergo  a  process  of  incomplete  segmentation,  only  a 
l^art  of  the  ovum  appearing  to  undergo  development,  and  they  are  accordingly  termed  mero- 
hla^tic  to  distinguish  them  from  those  (like  the  mammalian  ova)  in  which  the  yolk  or  nutri- 
tive material  is  everywhere  in  relatively  small  proportion  to  the  protoplasm,  the  ivhole  of 
which  undergoes  division  after  fertilization,  and  jjarticipates  in  the  formation  of  the  embryo 
{Jioloblastic  ova).  This  small  amount  of  nutritive  material  in  the  mammal  is  obviousl,y 
related  to  the  fact  that  the  mammalian  ovum  early  acquires  an  attachment  to  the  maternal 
system  from  which  it  is  then  able  directly  to  derive  its  nutriment,  whereas  the  meroblastic 
Dvum  of  oviparous  vertebrata  necessarily  contains  all  the  nutriment  required  by  the  developing 
bird,  reptile,  or  fish,  until  it  is  sufficiently  advanced  in  development  to  emerge  from  the  egg 
and  obtain  food  independently.  Although,  however,  the  mammalian  ovum  is  holoblastic,  it 
is  none  the  less  clear,  from  a  comparison  of  the  early  stages  of  its  develojpment  with  that  of 
the  bird,  that  the  ancestors  of  the  mammalia  must  have  had  ova  of  the  meroblastic  type. 

Balfour  has  further  conveniently  distinguished  between  those  ova  in  which  there  is  a  g-reat 
accumulation  of  nutritive  or  3'Olk  material  at  one  pole  (telolecithal  ova.  as  in  the  bird, 
reptile,  and  fish  amongst  vertebrates),  those  in  which  the  accumulation  of  yolk  is  in  the  middle 
of  the  ovum  {ccntrolemtlial  ova,  as  in  arthropods),  and  those  in  which  it  is  scattered  pretty 
equally  in  small  amount  throughout  the  protoplasm  without  any  very  marked  accumulation 

^  Purkinje  discovered  the  germinal  vesicle  in  the  bird's  ovum  in  1825  ;  that  of  mammals  was  first 
noticed  by  Coste  in  1833. 


MA'rL'l.'ATKiN    oK    TllK    OVUM, 


(iilrritliiil  ova.  as  in  iiiainnial>.  Aiiipliioxiis.  (rliiiinilcini.-).  It  is  clear  tliat  those  conditions 
of  arranjjcnu'Ut  of  tiic  proto-  and  tlciito-iilasni  wiiiiin  tin-  ovum  air  llic  main  factors  in  doter- 
niininj^  variations  in  the  [irocoss  of  scynu'ntatioii. 

Maturation  of  the  ovum.  Formation  of  polar  globules. —  llillicr  liefore 
its  c'sciipc  IVoiii  the  (iriiatijiii  Inlliclr,  dr  iiiiiiR'(li;iU'ly  iil'tcr,  I  lie  oviuii  undci'^foos  ii 
[n'culiiir  clianov.  pi\'i»iU'iit(irv  to,  luii  nevcrlliclcss  iiltoj^cthcr  iiid('i)C'iident  ol"  fci'ti liga- 
tion, wiiii'h  consists  of  a  process  of  iiiir(|iial  cell-division  or  jicnnination,  and  I'osults 
in  the  extrusion  from  the  vitellus  of  iwo  minute sjtherical  bodies  (fig.  8),  which  have 

Kili.    S. — Ovi'M    1)1.-    THi:     UAHBIT    IMIOM    TIIK     I'AI.I.hI'IAN    TIIIK,    TWKI.VKi 
llullis    AKTKK    lMPIU'.(iNAl'InN.       ( 1  iisrhdtt'.  ) 

On  tiie  zona  «,  spermatozda  are  seen,  and  (itliers  in  tlje   pciivitoliiiie 
space  ;  b,  the  puiar  yloljuk's. 

been  termed  the  jioliir  iilohnlcs  or  ilircrlli'c  (,)r/ii/sr/cs, 
from  a  supposition  that  their  presence  deterniincs  the 
jidle  at  which  the  first  seomentation  will  take  place 
should  the  ovum  become  fertilized.  It  is,  however, 
uncertain  whether  there  is  any  coustant  relationship  of 
this  kind,  but  it  is  none  the  less  clear  that  the  extrusion 
of  the  polar  globules  is  an  event  of  the  highest  importance  for  the  due  development 
(»f  the  ovum,  since  until  this  has  happened  the  ovum  ai)pears  to  be  incapable  of 
complete  fertilization  and  segmentation. ^  AVhat  is  actually  extruded  is  a  small  part 
of  the  nucleus  of  the  ovum,  or,  to  speak  more  precisely,  two  small  parts  of  its  nucleus 
in  succession,  probably  surrounded  by  a  very  thin  investment  of  protoplasm.  Prior 
to  this  extrusion,  the  germinal  vesicle  approaches  the  periphery  of  the  vitellus.  loses 
its  distinctness  of  outline,  and  after  passing  through  phases  which  are  charac- 
teristic of  a  nucleus  which  is  about  to  divide,  does  actually  undergo  a  division  into 
two,  the  one  part  being  extruded  into  a  space  (pcrivitelline),  which  has  become 
formed  in  consequence  of  the  shrinking  or  contraction  of  the  ovum,  and  the  other 
part  remaining  in  the  vitellus,  only,  however,  to  repeat  the  process  of  division,  and 
to  form  a  second  extruded  globule.  The  remainder  of  the  germinal  vesicle,  which  is 
now  termed  the  feniale  pronudpiis,  leaves  the  periphery  of  the  vitellus  for  a  situation 
nearer  to  the  centre,  where,  if  fertilization  should  supervene,  it  awaits  the  advent  of 
the  male  pronucleus,  which  is  formed  from  a  spermatozoon.  After  the  two  pro- 
nuclei have  come  together,  a  new  and  complete  nucleus  is  formed  by  their  conju- 
gation. 

The  actual  formation  of  polar  globules  has  not  hitherto  been  observed  in  the  human  ovum, 
althoug'h  there  is  no  doubt  whatever  that  it  takes  place.  In  the  rabbit  various  stag-es  in  the 
process  have  been  traced  by  E.  v.  Beneden  and  Rein,  and  it  has  also  been  noticed  in  other 
mammals.  But  the  details  of  the  process  have  been  made  outmost  preciselj'  (by  Fol.  Hertwij^'. 
and  others)  in  the  transparent  own  of  echinoderms,  and  more  recently  and  minutely  (by 
E.  V.  Beneden,  Carnoy.  Boveri.  Zacharias.  and  others)  in  Ascaris  meyalocephala,  a  thread-worm 
]>arasitic  in  the  horse,  in  which  all  the  changes  can  be  followed  in  one  and  the  same  ovum, 
or  the  various  phases  fixed  by  means  of  reagents  in  different  ova.  and  these  maj'  af tenvards  be 
stained  and  studied  with  the  utmost  minuteness.  The  successive  changes  in  such  ova  are 
represented  in  figs.  !)  and  10.  The  polar  globules  remain  visible  for  a  time  in  the  pcri- 
vitelline tinid.  and  are  even  .seen,  should  the  ovum  become  fertilized,  during  the  early  stages  of 
segmentation,  but  they  ultimately  disapjiear  and  are  not  known  to  take  any  further  part  in 
the  subsequent  changes  which  the  ovum  undergoes. 

The  fact  that  throughout  the  whole  animal  kingdom  the  extrusion  of  ])olar  globules  froni 
the  ovum  as  it  becomes  mature  is  almost  universal,  and  that  a  similar  process  has  also  been 
observed  to  occur  in  plants  indicates  the  grea,t  importance  of  the  phenomenon.  The  signifi- 
cance will  be  further  discussed  after  the  process  of  fertilization  of  the  ovum  has  been 
described. 

'  The  ovuui  may,  however,  receive  a  spermatozoon  before  the  eonipletion  of  tlie  formation  of  polar 
globules. 


10 


FORMATION    OF    POLAR    GLOBULES. 

B  C 


Fis.  9. 


-FoRilATIOJI    OF    THE    FIRST    POLAR    GLOBULE    IN    THE    EGG    OF    ASCAKIS    3IEGAL0CEPHALA. 

(v.  Gehuchten. ) 


A.  The  ovnm  with  the  germinal  vesicle  transfovmed  into  a  spindle  of  (achromatic)  fibrils  ;  from  the 
poles  of  the  spindle  other  fibrils  radiate  into  the  protoplasm.  At  the  equator  of  the  spindle  eight 
portions  of  chromatin  are  visible  ;  cs,  head  of  a  spermatozoon  which  has  previously  entered  the  ovum, 
and  is  becoming  transformed  into  the  male  pronucleus  ;    in,  gelatinous  membrane  of  the  ovum. 

B.  The  chromatin  particles  are  seen  separated  into  two  sets.  The  achromatic  fibrils  are  not  shown 
in  this  preparation.     The  ovum  is  considerably  shrunken. 

C.  Half  of  the  germinal  vesicle  is  extruded  into  a  perivitelline  space,  and  along  with  a  portion  of 
protoplasm  is  becoming  separated  otf  from  the  ovum  as  a  polar  globule.  The  extruded  half  includfs 
tour  of  the  chromatin  particles  ;  the  other  four  remain  in  the  ovum  ;  m',  membrane  dividing  the  polar 
globule  from  the  ovum. 


B 


Fia 


10. — Formation  of  the  second  polar  globule 
IN  ASCAKIS  megalocephala.     (Cai'noy.) 


A.  The  remainder  of  the  germinal  vesicle  (after 
extrusion  of  the  first  giobule,  g^)  has  again  become 
transformed  into  a  spindle  of  achromatic  fibrils, 
with  the  four  remaining  chromatin  particles  at  the 
equator  of  the  spinrl'e. 

B.  The  sijindle.  now  irregularly  Y-shaped,  is  seen 
approaching  the  surface  of  the  ovum  ;  (?',  first  polar 
globule ;  ns,  male  pronucleus  which  has  become 
formed  from  a  spermatozoon ;  p.  pole  of  spindle. 

C.  Extrusion  of  half  of  the  germinal  vesicle 
remainder. 

D.  Completion  of  the  jjrocess  ;  the  second  polar  globule,  g-,  is  now  separated  from  the  ovum  ;  it 
contains  two  of  the  chromatin  particles.  The  other  two  remain  in  what  is  left  of  the  germinal  vesicle, 
ii^   which  now  forms  the  female  pronucleus  ;  ns,  male  pronucleus  ;  rj^,  fii-sfc  polar  globule. 


FKjrrH.lZATlON. 


11 


Fertilization. — Tho  ovum,  after  its  fxpiilsitiii  IVuiii  Uie  CJraafian  lullicli;  is 
rccoivi'il  u]inn  the  fiinliriiiteil  end  ol'tlie  Fiilloi>iaii  tube.  The  linibriic  arc  covered  by 
a  ])roloiigati()n  ol'tho  ciUateil  linini:-  of  the  tul)e.  and  the  acti<ju  ol"  the  elHa  serves  to 
jiropcl  the  minute  ovum  into  and  aloii^-  i  he  tube  towards  the  uterus.  In  tliis 
jjassaije  it  may.  if  impretiiiation  have  oeeurred.  meet  with  the  spermatozoa,  one  or 
more  of  which  may  penetrate  the  zona  peUncida,  and  fertih'zc  the  ovum.  It  is 
possible  in  some  instances  for  fertihzation  to  occur  on  tlie  findjriated  extremity  of  the 
tube,  or  in  tlie  body  of  the  utei'us,  but  it  is  ])robable  that  in  most  ca.ses  it  happens 
in  the  tube  itself. 

It  is  probable  that  normally  only  a  single  spennatozoon  enters  the  vitellus.  If  it  should 
happen  that  two  or  more  enter,  normal  development  does  not  as  a  i-ule  occur.  Exceptions  to 
this  rule  have,  however,  been  recorded. 

The  changes  in  the  ovum  which  accompany  fertilization  have,  like  those  which 
result  in  the  formation  of  the  polar  globules,  been  studied  most  satisfactorily  in  the 
transparent  ova  ofechinoderms  and  in  Ascaris.    In  the  former  (fig.  11)  the  si)ermatozoa 


f.^r. 


iqn: 


Fig.  11. — Fertilization  op  the  ovum  of  an  echinoderm.     (Selenka. ) 

s,  spermatozoon  ;  m.pr,  male  pronucleus  ;  f.pr,  female  pronucleus. 

1.  Accession  of  a  spermatozoon  to  the  periphery  of  the  vitellus  ;  2.  Its  penetration,  and  the  radial 
disposition  of  the  vitelline  granules  ;  3.  Transformation  of  the  head  of  the  spermatozoon  into  the  male 
pronucleus  ;  4,  5.  Blending  of  the  male  and  female  pronuclei. 


may  be  seen  to  penetrate  the  gelatinous  investment  which  here  takes  the  place  of  a 
zona  pellucida,  and  the  head,  of  one  only  as  a  rule,  to  imbed  itself  in  the  ]ieriphery 
of  the  ovum,  which  becomes  slightly  protruded  at  the  point  of  contact.  According 
to  V.  Beneden's  account,  the  spermatozoon  always  enters  in  Ascaris  at  a  particular 
part  of  the  ovum  (polar  disc),  at  which  part  there  is  an  aperture  in  the  vitelline 
membrane  (micropyle).  When  once  it  has  passed  into  the  ovum,  this  aperture 
becomes  closed,  and  the  head  of  the  spermatozoon  rapidly  increases  in  size,  and 
acquires  the  appearance  of  a  nucleus  which,  in  contra-distinction  to  the  remains  of 
the  germinal  vesicle,  or  female  pronucleus,  is  termed  the  7nale  promicleus.  Soon 
it  leaves  the  periphery,  and  passes  towards  the  centre  of  the  ovum  in  the  direction 
of  the  female  pronucleus.  In  its  passage  through  the  protoplasm  it  appears  to 
exercise  a  peculiar  attraction  upon  the  granules  in  that  substance,  for  these  become 
arranged  in  its  vicinity  in  radiating  lines.  The  tail  of  the  spermatozoon  has 
in  the  meantime  disappeared,  whether  by  being  cast  off  or  by  blending  with  the 
protoplasm  of  the  ovum  has  not  certainly  been  made  out.  As  the  male  pronucleus 
approaches  the  female  pronucleus,  the  latter  moves  somewhat  to  meet  it,  and  pre- 
sently the  two  pronuclei  come  into  contact  and  together  fonn  a  new  nucleus,  com- 


vz 


FEKTILIZATIOX. 


plete  in  all  its  structure  and  functions.  AVitb  the  blending  of  tlie  two  pronuclei 
the  act  of  fertilization  is  completed,  and  the  ovum  is  now  capable  of  forming 
new  cells  by  division.  Since  the  head  of  the  spermatozoon  is  formed  from  the 
nucleus  of  a  seminal  cell,  part  of  which  appears  to  be  thrown  off  prior  to  the  com- 
plete maturation  of  the  spermatozoon  (Renson,  Brown),  and  the  female  pronucleus  is 
the  nucleus  of  an  egg  or  germ-cell,  part  of  which  has  been  removed  in  the  form  of  the 
polar  globules,  the  process  of  fertilization  may  be  described  as  consisting  essentially 
of  the  conjunction  of  part  of  the  nucleoplasm  of  a  sperm  cell  with  part  of  the  nucleo- 
plasm of  a  germ  cell,  the  result  being  the  production  of  a  complete  nucleus  end<;iwed 
with  active  properties  of  division  and  reproduction. 

Although,  as  has  been  already  stated,  the  chang-es  which  have  just  been  described  are  most 
clearly  to  be  seen,  and  have  been  most  completely  studied,  ia  the  ova  of  echinodei-ms  and 
Ascaris.  similar  processes  have  been  found  to  occur  in  most  if  not  ia  all  animals,  and  have 
even  been  made  out,  although  not  very  distinctly,  ia  mammals  (in  the  rabbit  by  v.  Beneden). 
There  is  no  doubt,  therefore,  that  the  phenomena,  of  fertilization  are  essentially  the  same 
throiig-hout  the  whole  animal  kingdom.  As  to  the  exact  details  of  the  process  there  is  still 
much  discrepancy  in  the  accounts  given  by  recent  observers.  Of  all  those  that  given  by 
V.  Beneden  of  the  process  of  fertilization,  and  of  the  subsequent  division  of  the  resulting 
nucleus  in  Ascaris,  is  the  most  explicit,  and  appears  to  negative  the  idea  of  a  complete  fusion 
taking  place  between  the  elements  of  the  pronuclei,  at  least  so  far  as  the  chromatin  is  concerned. 
According  to  this  account  (v.  fig.  12).  each  of  the  two  pronuclei  is  seen  to  possess,  previous  to  their 
conjunction,  two  short  chromatin  rods  {elinnnosoinc-s)  imbedded  in  clear  nuclear  matrix.  These 
rods  undergo  various  changes,  resulting  in  the  formation  of  a  skein  within  each  pronucleus 
(II..  III.),  but  eventually  the  skein  resolves  itself  into  two  Y-shaped  loops  or  filaments  (IT..  Y.). 
On  conjitnction  the  matrix  of  the  two  nuclei  may  appear  to  blend,  although  it  is  doubtful  if 
they  actually  fuse  together,  but  the  chi-omatin  filaments  retata  their  distinct  indiyiduality. 
The  nucleus  which  is  thus  formed  hj  the  conjunction,  contains,  therefore,  four  similar  Y-s)iaped 
chromatin  filaments,  which  now  split  longitudinally  (YL.  YII.).  and  after  being  aiTanged  for 
a  time  at  the  equator  of  the  now  spindle-shajDed  nucleus  (^^III.).  four  of  the  resulting  filaments 
pass  towards  the  one  pole,  and  form  CA-entually  the  chromatin  of  the  one  daughter  nucleus, 
and  foxu-  toAvards  the  other  pole,  eventually  forming  the  chi-omatin  of  the  other  daughter 
nucleus  (XI..  XII.).  It  is  stated  by  v.  Beneden  that  of  each  set  of  chromatin  iUaments.  or 
chromosomes,  which  thus  separate  from  one  another,  one  half  the  number  is  derived  from  the 
male  and  the  other  from  the  female  pronucleus.  If  this  is  the  case,  and  if  it  should  further 
be  shown  that  in  every  subseqitent  process  of  division  of  the  resitlting  cells,  the  chromatin 
filaments  of  the  daughter  cells  are  derived  half  from  male  chromatin  filaments  and  half 
from  female,  it  necessarily  follows  that  eA-ery  cell  nucleus  must  be  regarded  as  containing 
both  male  and  female  morphological  elements. 

Meaning'  of  the  polar  globules. — Theory  of  Minot. — The  question  of  the  hennaphroditism 
of  cells  Avas  first  raised  by  C.  S.  Minot  in  connection  Avith  the  separation  of  the  polar  globules. 
According  to  the  A'iew  advocated  by  Minot.  every  cell  which  results  from  the  division  of  a 
fertilized  ovum  is  hermaphrodite,  for  the  fertilized  ovum  is  formed  by  the  union  of  both  male 


Fig.  12. FORMATIOX    AND    CONJCGATIOX    OF    THE     PRONUCLEI    IN    ASCARIS    jrEGALOCEIHAIA. 

(E.  V.  Beneden.) 
/,  female  pronucleus  ;  m,  male  pronucleus  ;  ^),  one  of  the  polar  globules. 

I.  The  second  polar  globule  has  just  been  extruded  ;  both  female  and  male  pronuclei  contain  tAvo 
chromatin  particles  ;  those  of  the  male  pronucleus  are  beccming  transformed  into  a  skein. 

II.  The  chromatin  in  both  pronuclei  now  forms  a  skein. 

Ilrt.  The  skein  in  the  pronuclei  is  more  distinct.  Two  attraction -splieres,  each  with  a  central 
jjarticle,  united  by  a  spindle  of  achromatic  fibres,  have  made  their  appearance  near  the  pronuclei.  The 
male  pronucleus  has  the  remains  of  the  body  of  the  speiinatozoon  adhering  to  it. 

III.  The  pronuclei  are  enlarged  ;  the  skein  formation  of  the  chromatin  is  complete. 


III«. 


MEANING    Ob'    THE    I'OLAH    GLOBULES. 


13 


III/; 


r-.  '77. 


Illrt.  Ouly  the  female  pronucleus  is  sliown  in  this  figuie.  The  skein  of  this  is  contracted  and 
thickened.  The  attraction-spheres  are  near  one  side  of  the  ovum,  and  ;ire  connected  with  its  periphery 
by  a  cone  of  fibres  forming  a  polar  circle  p. c.  ;  c.c.  equatorial  circle. 

Illi.  The  pronuclei  have  approached  one  another,  and  the  spindle -system  is  now  aiTanged  across 
their  common  axis. 

IV.  Conti-action  of  the  skein  and  formation  of  two  V-shaped  chromatin  filaments  in  each  pronucleus. 

V.  The  V-shaped  chromatin  filaments  are  now  quite  distinct ;  the  male  and  female  pronuclei  are  in 
close  contact. 

VI.,  VII.  The  V-shaped  filaments  are  splitting  longitudinally  ;  their  structui-e  of  fine  granules  of 
chromatin  is  apparent  in  VII.,  which  is  more  highly  magnified.  The  conjugation  of  the  pronuclei  is 
apparently  complete  in  these  figures,  hut  according  to  v.  Beneden's  description,  the  outlines  of  both 
can,  under  favourable  conditions,  be  still  made  out.  The  attraction  spheres  and  achromatic  spindle, 
although  present,  are  not  shown  in  IV.,  V.,  VI.  and  VII. 

VIII.  Equatorial  arrangement  of  the  four  cliromatin  loops  in  the  middle  of  the  now  elongated  ovum  : 
the  achromatic  substance  forming  a  spindle-shaped  system  of  granules  with  fibrils  radiating  from  the 
poles  of  the  spindle  (attraction-spheres)  into  the  protoplasm  ;  commencing  division  of  the  ovum  into 
two  cells. 

IX.  Shows  diagrammatically  the  commencing  separation  of  the  chromatin  filaments  of  the  con- 
jugated nuclei,  and  the  system  of  fibres  radiating  from  the  attraction-spheres.  p.c.  polar  ciicl^  : 
f.c.  equatorial  circle  ;  c.c.  central  particle. 

X.  Further  separation  of  the  chromatin  filaments.  Each  of  the  central  particles  of  the  attraction- 
spheres  has  divided  into  t)vo. 

XI.  The  chromatin  filaments  are  becoming  developed  into  the  skeins  of  the  daughter  nuclei.  These 
are  still  united  by  achromatic  fibres.     The  protoplasm  of  the  ovum  is  becoming  divided. 

XII.  The  daughter  nuclei  exhibit  a  chromatin  network.  Each  of  the  attraction-spheres  has  divided 
into  two,  which  are  joined  by  achromatic  fibres,  and  are  connected  with  the  periphery  of  the  cell  in 
the  same  manner  as  the  parent  sphere  shown  in  Ilia. 


14  liECJ^NT    LITEKATUllE    UF    THE    OVUM. 

and  female  elements,  and  every  one  of  its  descendants  must  also  contain  a  certain  proportion 
of  each.  For  the  sexual  conjuffation  of  two  cells  it  is  assumed  to  be  necessary  that  the  on& 
should  get  rid  of  the  male  elements,  and  retain  only  the  female,  and  that  the  other  should  be 
exclusively  male.  This  is  effected  in  the  one  case,  according  to  Minot,  by  the  extrusion  of  the 
polar  globules,  which,  in  this  view,  represent  the  male  element  of  the  originally  hermaphrodite 
generative  cell,  so  that  when  they  are  extruded  this  remains  v^hoUy  female  ;  in  the  other  case 
there  is  also  a  separation,  and  the  separated  part  becomes  disii  tegrated,  leaving  only  the  male 
portion,  or  spermatozoon — the  separated  part  in  this  case  represents,  therefore,  the  female 
element  of  the  generative  cell. 

Theory  of  Weismann. — Minot's  theory  was  adopted  by  Balfour,  who  looked  upon  the 
formation  of  polar  cells  as  having  been  acquired  by  the  ovum  for  the  expi-ess  purpose  of  pre- 
venting parthenogenesis.  According  to  this  viev  no  polar  globules  should  be  formed  in  parthe- 
nogenetic  ova,  and  it  was  believed  by  both  Minot  and  Balfour  that  they  would  not  be  found 
to  occur.  It  has,  however,  since  been  discovered  by  Weismann  and  Blochmann  that  parthe- 
nogenetic  ova  do  extrude  one  polar  globule,  although  the  ordinary  ova  of  the  same  animal 
extrude  two. 

It  is  clear  that  this  fact  renders  a  modification  necessa.ry  in  the  view  advocated  by  Minot 
and  Balfour.  Such  modification,  or  substitute,  as  it  may  perhaps  more  appropriately  be 
termed,  has  been  furnished  by  Weismann  in  his  theory  of  heredity  (  Ve7-erhungstheorie).  This 
theory  assumes  that  every  animal  and  vegetable  cell  contains  two  different  kinds  of  living 
substance.  These  are  termed  by  Weismann  the  iiuclear  plasma  and  the  nutritive  2>lasma.  The 
former  is  endowed,  with  germinative,  directing  and  hereditary  functions,  the  latter  with, 
assimilation  of  food  and  the  more  purely  physical  functions  (contraction,  nerve-conduction, 
secretion,  &c.),  but  these  functions  are  assumed  to  be  carried  out  under  the  direction  of  the 
nuclear  plasma.  The  nuclear  plasma  is  further  supposed  by  Weismann  to  consist  of  two  sub- 
stances, viz.,  a  germinal  2)la.sma  which  is  the  primitive  form,  and  which  alone  is  endowed  with 
heredity,  and  a  Idstogenetic  jilasma  which  has  been  derived  from  the  germinal  plasma,  and  which, 
controls  the  division,  growth,  and  differentiation  of  the  cell.  Fertilization  consists  in  the 
bringing  to  the  ovum  of  a  certain  amount  of  germinal  plasma  from  a  different  individual,, 
and  Weismann  assumes  that  it  is  necessary  for  the  ovum,  prior  to  fertilization  and  develop- 
ment, to  get  rid  both  of  its  old  histogenetic  plasma  and  of  so  much  germinal  plasma  as  may 
be  brought  to  it  by  the  spermatozoon,  and  that  it  effects  this  by  the  extrusion  (1)  of  one 
(histogenetic)  polar  globule,  (2)  of  the  other  (germiual)  globule.  If  this  is  what  happens,  the 
primitive  or  germinal  plasma  is  never  wholly  eliminated  from  the  ovum,  so  that  it  may  be 
looked  upon  as  transmitting  all  the  accumulated  ancestral  characters  which  have  been  derived 
from  the  vast  number  of  its  predecessors.  A  portion  is,  however,  got  rid  of  in  the  form  of 
the  second  polar  globule,  and  what  remains  is  not  necessarily  of  quite  the  same  constitution  in 
every  case,  nor  is  the  portion  of  germ  plasma  brought  by  the  spermatozoon  necessarily  always 
similar  :  these  differences  in  the  germinal  plasma  of  the  fertilized  ovum  may  account,  accord- 
ing to  Weismann,  for  the  individual  differences  which  occur  in  the  progeny.' 

Weismann  and  Ischikawa  have  shown  that  in  some  animals  the  segmentation  of  the  ovum 
may  have  advanced  through  one  or  two  stages  before  the  entry  of  a  spermatozoon.  In  this 
case  the  spermatozoon  (male  pro-nucleus)  blends  with  the  nucleus  of  only  one  of  the  cells 
which  have  resulted  from  the  segmentation.  Probably  the  sexual  cells  are  the  ultimate  result 
of  this  conjugation. 

BE  CENT    LITERATURE. 

Beneden,  E.  v.,  Recherches  sur  la  maturation  de  I'oeuf  et  la  fecondation.  Arch,  de  biolog.,  iv.,. 
1884  ;  Fertilization  and  Segmentation  in  Ascaris  Megalocephala,  Journal  of  Microscopic  Science,  1888 
(Bulletin  de  racademie  r.  des  sciences  de  Belgique,  1887,  t.  xiv.)  ;  Sur  la  fecondation  chez  I'ascaride 
m.egaloc6phale,  Anatomischer  Anzeiger.     Jahrg. ,  iii.,  1888. 

Beneden,  E.  v.  et  Neyt,  A.,  Nouvelles  recherches  sur  la  fecondation  et  la  division  mitosique 
chez  I'ascaride  megalocephale  (Bullet,  de  I'acad.  royale  des  sciences  de  Belgique,  3  ser.  t.  xiv.,  1887). 

Blochmann,  F.,  t/eber  die  Richtungskorper  hei  Insekteneiern,  Morphol.  Jalirbuch,  Bd.  xii.,  1887. 
Also  in  Morph.  Jahrb.  xv.,  1889.  and  Verhandl.  d.  naturhist.  med.  Vereins  zu  Heidelberg,  1888. 

Boveri,  T.,  Zellenstudien,  H.  1,  Die  Bildung  der  Richstungskorper  bei  Ascaris  megalocephala  und 
Ascaris  lumhricoides,  Jenaische  Zeitschr,  f.  Naturwiss.,  Bd.  xiv.,  1887  ;  Zellenstudien.  U.  2.  Die 
Befruchtung  und  Theilung  des  Eies  von  Ascaris  megalocephala,  Jena.  Zeitschr.,  1888  ;  Zellenstudien. 
H.  3.  TJeber  das  Verhal.ien  der  chromatischen  Kernsubstanz  bei  der  Bildung  der  Richtungskorper  «. 
bei  der  Befruchtung,  Jena.  Zeitschr.,  1890, 

Biitschli,  O.,  Gedanken  ilber  die  morphologisehe  Bedeutung  der  sogenannten  Richtungskorperehetiy 
Biolog.  Centralbl.,  Bd.  iv.,  1884. 

^  It  is  difficult  to  do  any  justice  to  Weismann's  theory  in  a  short  space,  and  the  above  is  to  be  tatem 
as  only  furnishing  a  rough  sketch  of  its  general  outline.  For  a  complete  account  the  reader  is 
referred  to  Weismann's  publications  upon  the  subject  (see  Literature). 


KKCENT    LITERATUKIi    OF    THE    UVUM.  15 

Caldwell,  W.  H.,  The  Embryology  of  Monotremata  and  MarmjiiaUa,  part  i.,  PhiJosopliical 
Transactions  of  tlie  Uoyal  Society  of  London  for  tho  year  1887. 

Carnoy,  J.  B.,  Les  ijlohes  polaires  de  Vascaris  clavata.  La  Cellule,  t.  iii.,  1887  ;  La  v^siculr 
germinatire  ft  les rilnhes  jv>laires  cluz  ijuchiues  uimatoden.  La  Cellule,  t.  iii.,  1887  ;  La  visictile  gennina- 
tive  et  les  iitobults  polaires  de  I'Ascaris  viegalocepholn,  La  Cellule,  t.  ii.,  18S7  ;  Some  Remarks  on  the 
recent  Restarc/ies  of  Zuchar'ms  and  Boveri  upon  the  Fraindation  of  A  scar  is  megaloceplmUi.  Report  ol 
the  .^>7tli  meeting  of  the  British' Association  for  the  Advancement  of  Science  at  Manchester,  1887. 

Cunningrham,  J.  T.,  E.  v.  Benedens  Researches  on  the  Maturation  ami  Fecundation  of  the  Ovum, 
QiMit.  .'ourn.  of  iMicrosc.  Science,  Ji\n.,  1885. 

Qehuchten,  A.  v.,  Nouvclles  observations  sur  la  visicide  germinative  et  les  globules  polaires  de 
I'Ascarii  inegnhicephala,  Anat.  Anz.,  No.  25,  1887. 

Qriitzner,  P.,  Physioltgische  Unter&uchungen  ilber  die  Zeugung,  Deutsche  medic.  Wochenschr., 
1884. 

Henking*,  H.,  Vcber  Rcductionsteilung  dcr  Chromosomen  in  den  Samenzellen  von  Insccten. 
Monthly  International  Journal  of  Anatomy  and  Physiology,  vol.  vii.,  1890. 

Hertwig-,  Oscar,  und  Richard,  Ueber  den  Befruchtungs-  und  Theilungsvorgaruj  des  thierischen 
Eies  untcr  dem  Einjluss  dusserer  Agentien,  Jena.  Zeitschr.  f.  Naturwiss.,  Bd.  xx.,  1887. 

Hertwig,  R.,  L'eber  den  Einfluss  des  Cldoralhydrats  auf  die  inneren  Befruchtungserscheinungen, 
Anatoin.  Anzeiger,  1886. 

Kolliker,  A.  v..  Das  Karyoplasma  und  die  Vererbung,  eine  Kritik  der  Weisrnann' schen  Theorie 
von  der  Continuit'dt  des  Keimplasma,  Zeitschr.  f.  wissensch.  Zoologie,  Bd.  xliv.,  1886. 

Kultschitzky,  N.,  Die  Befruchtungsvcrrgdnge  bci  Ascaris  megalocephala,  Arch.  f.  microsc.  Aoat., 
Bd.  xxxi. ,  18SS  ;  Ueber  die  Eireifung  und  die  Befruchtungsvorgdnge  bei  Ascaris  marginata,  Ebendas., 
Bd.  xxxii.,  ISSS. 

Kupffer,  C. ,  Die  Befruchtung  des  Forelleneies,  Bayerische  Fischerei-Zeitung,  1886. 

Minot,  Ch.  S.,  Theorie  der  Oojioblasten,  Biol.  Centralbl.,  Bd.  ii.,  1882. 

Mondino,  C,  e  Sala,  L.,  Sur  les  phdnom&nes  de  maturation  et  de  fdcondation  dans  les  oeufs  des 
ascarides.     Arch.  ital.  de  biol.  xii.,  1SS9. 

Nagel,  W.,  Das  menschliche  M,  Arch.  f.  micr.  Anat.,  Bd.  xxxi.,  1888. 

Nussbaum,  M.,  Ueber  die  Verdnderungender  Geschlechtsprodiictebis  zur  Eifurchung.  Ein  Beitrag 
tur  Lehre  der  Vererbung,  Arch.  f.  mikrosk.  Anat.,  Bd.  xxiii.,  1884  ;  Bildungu.  Anza/d  d.  Richtungsk. 
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Saugethierei,  Archiv  f.  mikr.  Anat.,  1883. 

Retzius,  Gr.,  Zur  Kenntniss  v.  Bau  d.  Eierstockseies,  die,  Hygiea,  Festband,  1889. 

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Sehlen,  D.  v. ,  Beitrag  zur  Frage  nach  der  Mikropyle  des  Sdugethiereies,  Archiv.  fiir  Anat.  a. 
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Sheldon,  L.,  The  Maturation  of  the  Ovum  in  Peripatus,  Quart.  Joum.  of  Microscopical  Science, 
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"Weismann,  A,,  n.  Ischikawa,  C,  Ueber  die  Bildung  der  Rlchtungskorper  bei  thierischen 
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1889-90  ;   Ueber  imrtidle  Befruchtung,  Freiburg  i.  Br.,  1888  ;  Biol.  Centralbl.,  Bd.  viii.,  1888. 

Zacharias,  O.,  Neue  Untersuchungen  iiber  die  Copulation  der  Geschlechtsproducte  und  den 
Befruchtung svor gang  bei  Ascaris  megalocephala,  Arch.  f.  mikrosk.  Anat.,  Bd.  ixx.,  1887  ;  Ueber  die 
Bildung  der  Richtungskorper  bei  thierischer  Eiern,  Biol.  Centralb.,  Bd.  viii.,  1888  ;  Ueber  Abweichung en 
vom  Typus  der  Conjugation  der  Geschlechtskerne,  Anat.  Anzeiger,  iii.,  1888. 

Ziegler,  E.,  Konnen  erworhene  pathologische  Eigenschaften  vererbt  werden  und  wie  entstehen 
erbliche  Krankheiten  und  Missbildungent  Beitrage  zur  pathoL.  Anat.  u.  Physiol.,  herausgegeben  von 
^egler  and  Nauwerk,  1886. 


16 


SEGMENTATION'    OF    THE    OVUM. 


EAELY  CHANGES  IN  THE  OVUM   CONSEQUENT  ON  FERTILIZATION— SEGMENTATION 
FORMATION  OP  THE  BLASTODERM— THE  PRIMITIVE   STREAK  AND  GROOVE. 

Segmentation  of  the  ovum. — Immediately  after  the  completion  of  the  process 

of  fertilization,  the  ovum  begins  to  show  signs  of  division  into  two  cells  or  segments. 
The  division  is  preceded  by  the  formation  of  a  spindle-shaped  system  of  achromatic 


Fig,    13. — First    .stages 

OF  SEGMENTATION  OF  A 
MAMMALIAN  OVUM: 
SEMI-I>IAGRAMMAT  IC. 

(Drawn  by  Allen  Thom- 
son after  E.  v.  Beue- 
den's  description.) 

z.p,  zona  pellucida  ; 
'p.gl,  polar  globules  ;  a, 
division  into  two  .seg- 
ments ;  ect,  larger  and 
cl  earer  segment  ;  ent, 
smaller,  more  granular  seg- 
ment ;  b,  stage  of  four  seg- 
ments ;  c,  eight  segments, 
the  ectomeres  partially 
enclosing  the  eutomeres  ; 
d.e,  succeeding  stages  of 
segmentation  showing  the 
more  rapid  division  of  the 
clearer  segments  and  the 
enclosnre  of  the  darker 
segments  by  them. 

fibres  and  by  changes  in  the  nucleus  which  are  similar  to  those  which  take  place  in 
the  division  of  an  ordinary  cell  (v.  Histology).  According  to  v,  Beneden's  observa- 
tions in  Ascaris  these  changes  occur  in  each  of  the  two  pro-nuclei  (fig.  12),  and 
one-half  the  number  of  resulting  Y-shaped  filaments  then  passes  from  each  to  form 
€ach  daughter  nucleus,  which  thus  contains  male  and  female  chromatin  elements  in 
equal  amount.  Each  of  the  two  segments  which  are  thus  formed  speedily  again  divides 
in  the  same  manner,  so  that  four  cells  or  segments  now  occupy  the  interior  of  the  ovum. 
By  a  further  process  of  binary  division  eight  cells  are  formed,  then  sixteen,  thirty-two, 
and  so  on  until  the  originally  simple  ovum  is  eventually  subdivided  into  a  large 
number  of  small  segments,  each  of  which  is  a  nucleated  cell,  which  are  aggregated 
into  a  solid  spherical  mass,  not  much  larger  than  the  original  ovum,  and  known  as 
the  mulberry  mass.  The  cells  are  not  similar  throughout,  for  those  at  the  surface 
are  clearer  and  less  granular  than  those  which  occupy  the  interior  of  the  mass. 
According  to  v.  Beneden's  observations  in  the  rabbit  and  bat,  this  difference  in  the 
appearance  of  the  cells  is  traceable  even  in  the  first  pair  of  daughter  cells,  one  of 
which  is  larger  and  clearer  than  the  other,  which  is  darker  and  more  granular  (fig. 
13,  a)}  The  cells  or  segments  which  result  from  the  division  of  each  of  these  retain 
their  respective  characters,  and  since  the  clearer  cells  divide  somewhat  more  rapidly 
than  the  darker  ones,  there  are  for  a  time  at  certain  stages  of  the  process  of  segmen- 
tation more  of  the  clear  cells  ;  thus,  at  one  stage  there  are  eight  clear  cells,  and  only 
four  darker  ones,  the  latter  having  not  yet  undergone  division,  and  later  there  are 
sixteen  clear  cells  and  only  eight  darker  cells,  for  the  same  reason.  Further,  it  is 
found  that  as  the  segmentation  proceeds  the  clearer  cells  occupy  the  superficial  part 
of  the  ovum  and  almost  entirely  enclose  the  granular  cells  which  fill  the  interior 
(fig.  14,  a).' 

'  This  statement  has  been  denied  by  Kolliker  and  other  observers,  who  assert  that  there  is  no 
difference  in  the  size  and  appearance  of  the  first  segments. 

-  Numerous  experiments  have  been  made  in  recent  years  with  the  view  of  determining  whether  the 
first  cells  which  result  from  the  division  of  the  ovum  have  specific  characters  and  give  vise  to  specific 


FORMATION    OF    THE    BLASTObEKM. 


17 


Tlio  ovum  next  undergoes  a  rapid  increase  in  size  owin^  to  the  segregation  of 
fluid  between  the  clear  suix'rficial  layer  of  cells  and  the  enclosed  granular  segments, 
which  thus  become  separated  from  one  another  except  at  one  part  (fig.  14,  h).  At  the 
same  time  the  superficial  cells  multiply,  and,  becoming  flattened  out  like  a  pavement 
epithelium,  form  a  membrane  enclosing  the  contained  fluid.  The  ovum  is  now  a 
thin-walled  transi)arcnt  sac,  occupied  by  fluid  and  enclosed  by  two  memljranes,  one 


Fig.  14. — Sections  of  the  ovcm  of  the  rabbit 

PCRIXO  THE  LATER  STAGES  OF  SEGMENTATION, 
SHOWING  THE  FORMATION  OF  THE  BLASTODER- 
MIC VESICLE.     (E.  V.  Beneden. ) 

rt.  Section  showing  the  enclosure  of  darker  cells, 
ent,  by  clearer  cells,  tct ;  h,  more  advanced  stage 
in  which  fluid  is  beginning  to  accumulate  be- 
tween the  inner  and  outer  cells,  the  former  com- 
pletely enclosed  ;  c,  the  fluid  has  much  increased, 
so  that  a  large  space  separates  inner  from  outer 
cells  except  at  one  part  ;  d,  blastodermic  vesicle,  its 
wall  formed  of  a  layer  of  flattened  cells,  with  a 
patch  of  dark  granular  cells  adhering  to  it  at  one 
part  ;  z.p.,  zona  pellucida. 

being  the  thinned-out  zona  pellucida  and 

the  other  the  epithelial   membrane  just 

mentioned.     Adherent  to  one  part  of  the 

inner    surface  of  this   membrane  is  the 

little  mass  of  dark  granular  cells  which  formerly  occupied  the  whole  interior  of  the 

mulberry  mass,  and  these  cells  give  to  the  part  of  the  ovum  where  they  occur  a 

darker  appearance,  when   it  is  viewed  by  transmitted  light.     At   this  stage  of 

development  the   ovum  has  been  termed  the  iI(j^Jodernncj.'eside  (fig.  14,  c,  d), 

although  the  actual  Ijlastodemi  is  not  yet  formed. 

Formation  of  the  blastoderm. — Soon  the  granular  cells  are  found  to  be  no 
longer  accumulated  into  a  small  mass  but  to  be  spreading  out  in  the  form  of  a 
lenticular  patch  over  the  inner  surface  of  the  vesicle.  As  this  extension  proceeds 
the  innermost  cells  separate  off  as  a  distinct  layer,  the  separation  starting  from 
the  centre  and  progressing  outwards. 

A  section  through  the  middle  of  the  ovum  now  shows  three  layers  (fig.  15)  :  an 
outer,  which  is  the  epithelial  membrane  of  the  blastodennic  vesicle  (Rauber's  layer) ; 
an  inner,  which  may  be  termed  the  pnmiJu-e  enUJdJ'rm,  from  the  fact  that  it  becomes 
the  innermost  layer  of  the  blastoderm,  and  an  ill-defined  middle  stratum  of  somewhat 

portions  of  the  organism  or  not.  The  general  result  of  these  has  been  to  show  that,  although  under 
certain  conditions  one  of  the  two  cells  resulting  from  the  division  of  the  ovum  may  on  the  destruction 
of  the  other  develop  into  a  semi-embryo  (this  in  the  frog  by  Eoux),  nevertheless"  completely  isolated 
cells  can  develop  into  full  embrj-os  (Driescn  in  Echinus,  Wilson  in  AmpMoxus,  0.  Schidtze'  in  frog). 
For  a  detailed  account  and  literature  of  this  subject  see  0.  Hertwig,  Lehrb.  d.  Allg.  Anatomic  °u. 
Physiol.  1898,  Bd.  ii. 


18 


FORMATION  OF  THE  BLASTODERM. 


granular  cells,  which  represents  the  remainder  of  the  inner  granular  mass  of  the 
blastodermic  vesicle  after  the  separation  of  the  subjacent  layer.  The  three 
layers  were  believed  by  v.  Beneden  to  represent  the  three  permanent  layers  of  the 
blastoderm.  But  it  has  been  conclusively  shown  (by  Rauber  and  Kolliker  in  the 
rabbit,  and  by  Lieberkiihn  and  Heape  in  the  mole)  that  the  middle  stratum  of  this 
stage  of  development  is  not  the  permanent  middle  layer  of  the  blastoderm,  for  it  is 


Fig.  L5. — Section  of  part  of  the  blastodermic  a'^esicle  op  the  rabbit  at  six  days.      (From 

E.  van  Beueden. ) 

a,  upper  layer  (Raiiber's  cells)  forming  with  h,  the  primitive  ectoderm  ;  c,  primitive  entoderm. 


Fig.  IG. — -A  section  through  part  of  a  bilajiinar  blastoderm  of  the  cat.     (E.  a.  S.) 
cct,  primitive  ectoderm  ;  ent,  primitive  entoderm  ;  z.p.,  thinned-out  zona  jjcllucida. 

before  long  converted  into  a  layer  of  columnar  cells  which  becomes  closely  applied  to, 
and  soon  blends  with,  Rauber's  layer,  so  that  the  two  now  form  but  a  single  stratum, 
which  may  be  denominated  the  jjrimiiive  ectoderm. 

Kolliker  describes  the  cells  of  Rauber's  layer  as  undergoing-  a  kind  of  atrophy  and  gradual 
disappearance,  taking  no  part  in  the  fonnation  of  the  isrimitive  ectoderm.  The  observa- 
tions of  Lieberkiihn  and  Heape,  on  the  other  hand,  tend  to  support  the  view  -which  has  been 
given  in  the  text. 

Both  layers,  but  especially  the  primitive  ectoderm,  are  somewhat  thickened  near 
the  middle  of  the  ovum  over  a  circular  or  oval  area,  which  appears  slightly  darker 
than  the  rest  of  the  ovum  when  this  is  viewed  by  transmitted  light  :  it  is  known  as  the 
emtryonic  area  (fig.  17).  The  entoderm  does  not  for  a  long  time  form  a  complete  invest- 
ment to  the  blastodermic  vesicle,  for  as  we  have  seen  it  commences  to  form  near  the 
centre  of  the  ovum,  and  only  gradually  grows  round  within  the  epithelial  investment, 
so  that  it  terminates  peripherally  by  a  free  border.  In  most  mammals  which  have 
been  investigated,  it  has  not  completely  enclosed  the  ovum  when  the  mesoblast  has 
begun  to  form,  but  in  the  cat  its  growth  appears  to  progress  more  rapidly,  so  that, 
for  a  time,  the  blastodermic  vesicle  has  two  complete  and  distinct  epithelial  invest- 
ments. Whether  complete  or  incomplete,  the  two  layers  together  constitute  what  is 
known  as  the  lUanmuir  Masiods&ii  (fig.  16),  the  formation  of  which  marks  a  distinct 
■stage  in  the  development  of  all  the  metazoa. 

But  the  blastoderm  does  not  long  remain  in  the  bilaminar  condition.  In  the  rabbit 
and  mole,  and  probably  in  most  mammals,  long  before  the  primitive  entoderm  has 
completely  extended  itself  around  the  ovum,  there  occurs  a  considerable  thickening 
of  the  primitive  ectoderm  at  one  end — the  posterior — of  the  somewhat  oval  embryonic 
area.  This  thickening  has  at  first  a  crescentic  form,  with  the  concave  edge  looking  for- 
wards, and  from  the  middle  of  this  edge  a  longitudinal  thickening  extends  for  a  certain 
distance  towards  the  centre  of  the  embryonic  area.  The  thickening  is  produced  by  a 
proliferation  of  the  cells  of  the  primitive  ectoderm,  and  its  consequent  downgrowth 
towards  the  primitive  entoderm,  and  it  is  visible  when  the  ovum  is  viewed  from 
above  by  transmitted  light,  as  a  streak  or  shadow  which  is  known  as  the  p-imitive 


FOIIMATION    OF    THK    BLASTODERM. 


19 


fifnak  (f[»i.  1 S,  1  <)).  Almost  as  soon  as  the  i)riiiiitive  streak  has  heconie  fully  formed,  it 
may  be  observed  to  be  Sfored  alon^-  its  leiiirth,  except  at  the  anterior  end  which  is  that 
directed  towards  the  centre  of  the  embryonic  area,  by  a  narrow  groove — the 
/in'mifire  (/rooi'e.  The  proliferating  primitive  ectoderm  comes  into  close  relationship 
below  this  groove  with  the  primitive  entoderm,  and  t!ie  two  may  be  partly  or 


'^►•S 

il 


it:  .    V 


Fig.  17. — Embryosic  area  of  mole  immediately  prior  to  appearance  ur  primitive  streak  and 

FORMED   OF   TWO    LAYERS   ONLY, 

Fig.  IS.  — Embryonic  area  of  the  mole  showing  the  primitive  streak  and  groove  ending 

POSTERIORLY    IN    A    CRESCENTIC    THICKENING. 

The  area  is  bilaminar  in  front,  trilaminar  in  the  posterior  half. 

Fig.    19.— A    SOMEWHAT    LATER    STAGE    IX    WHICH    THE    PRIMITIVE    STREAK     REACHES    TWO-THIRDS    OP    THB 
LENGTH    OF    THE    EMBRYONIC    AREA,    AND    ENDS    BEHIND    IN    A    KNOB    OR    THICKENING. 

Figs.  17,  18,  and  19  are  copied  from  Heape.      They  are  magnified  49  times. 

entirely  blended,  but  the  union  is  closest  at  the  anterior  end  of  the  primitive  streak 
where  a  continuous  column  of  cells  unites  the  primitive  ectoderm  and  entoderm, 
so  that  the  two  layers  are  here  indistinguishable. 

The  proliferation  of  the  cells  of  the  primitive  streak  subsequently  proceeds  chiefly 
at  the  sides  of  the  primitive  gi'oove,  and  the  cells  which  are  produced  by  this  proli- 
feration extend  themselves  laterally  between  the  ectoderm  and  entoderm  to  form  a 


Fig.  20,  A.  and  B. — Views  of  the  embryonic  area  of  the  rabbit   showing   two   stages  is   thb 

EXTENSION    OF    THE    3IES0 BLAST.       (Kolliker.) 

In  A.  the  mesoblast  extends  on  either  side  of  the  primitive  streak  over  the  posterior  part  of  the 
cmhryonic  area  and  also  behind  the  primitive  streak  bevond  the  limits  of  that  area. 

In  B.  the  mesoblast  extends  over  a  circular  area  which  surrounds  the  embryonic  area.  The  em- 
bryonic area  is  also  ti-ilaminaiv  except  in  the  middle  line  in  front  of  the  primitive  streak. 


20 


FOEMATION    OF    THE    BLASTODERM. 


third  or  intermediate  layer.  This  is  mainly  derived,  as  was  first  pointed  out  by 
KoUiker,  from  the  primitive  ectoderm  of  the  groove,  but  since  in  this  situation  the 
two  primary  layers  become  eventually  more  or  less  blended,  it  is  probable  that  the 
primitive  entoderm  cells  also  take  part  in  its  formation,  although  this  part  is  in  the 
mammal  evidently  a  subordinate  one.     It  is  further  maintained  by  many  embry- 


Fig.  21.— Section  across  the   posterior   end  of   the   embryonic   area_  of  a  rabbit  at  the  time 

OF    THE    first    SIGN    OF    A    PRIMITIVE    STREAK.       (KolUker.  ) 

ep,  epiblast  ;  ax,  its  axial  part  undergoing  proliferation  (this  is  shown  by  the  karyokinetic  figures,. 
k) ;  me,  mesoblast  becoming  derived  from  the  proliferating  axial  epiblast ;  hy,  hypoblast. 


Yia,  22. Longitudinal  section  through  the  middle  line  op  part  of  an  embryonic  area  (mole)  in- 

which  the  primitive  streak  has  begun  to  form.     (Heape.) 
The  blastoderm  is  perforated  in  front  of  the  (short)  primitive  streak  (?  blastopore,  hlxi) ;  a  few  meso- 
blast cells  are  seen  anterior  to  the  perforation  ;  ep,  epiblast ;  hy,  hypoblast  ;  p.sk,  primitive  streak. 


Fig.  23.— Two  sections  across  the  embryonic  area  of  a  blastoderm  at  the  stage  shown  in 

fig.  19.     (Heape.) 

A.  Section  across  the  anterior  end  of  the  primitive  streak  and  groove. 

B.  Section  across  the  posterior  enlargement  of  the  primitive  streak.  The  epiblast  and  hypoblast 
are  seen  to  be  united  along  the  primitive  streak,  p.sk  ;  laterally  the  mesoblast,  m..  the  cells  of  which 
have  grown  out  from  the  uniting  column  of  axial  cells,  separates  the  two  primary  layers. 

p.gr,  primitive  groove  ;  ep,  epiblast ;  Jiy,  hypoblast  ;  m,  mesoblast. 


GASTRULAlloX  OF  VERTEBRATES. 


21 


Pig.    2-t. — Four    stages   IX    the    development   op   AMPHIOXUS    illustrating    the    FORMiTION    OF    TUE 

GASTRULA.     (Hatscbek. ) 

I.  Spherical  blastodenn  ;  tlie  cells  at  the  lowei-  pole  are  larger  thau  the  others,  and  filled  with 
granules. 

II.  Invagination  of  the  lower  pole  producing  a  cupping  of  the  vesicle. 

III.  Completion  of  the  invagination  ;  the  blastoderm  is  now  bilaminar,  and  form.s  a  cup  with 
marrowed  mouth,  the  blastopore,  hi,  and  a  double  wall  of  epiblast,  cp,  and  hyijoblast,  hij  (or  primitive 
ectoderm  and  primitive  entoderm). 

IV.  The  ovum  is  now  elongated  ;  the  cavity  of  the  gastrula  forms  a  primitive  alimentary  canal,  the 
orifice  of  which  is  the  blastopore,  which  is  directed  dorsally.  Extending  from  this  along  the  dorsal 
■surface  (right  in  the  figure)  a  shallow  groove  is  seen  in  optical  section  :  this  is  the  rudiment  of  the 
nervous  system. 


oiogists  that  cells  from  the  lateral  parts  of  both  primary  layers  are  added  to  the 
intermediate  layer,  and  assist  in  its  extension.  According  to  the  observations  of 
Bonnet  in  the  sheep,  there  is  an  addition  to  the  middle  layer  from  the  peripheral 
(thickened)  portion  of  the  hypoblast  ;  this  has  been  long  held  to  be  the  case  with 
the  blastoderm  of  the  bird,  and  the  cells  thns  derived  (parablastic)  have  l)een 
■considered  to  have  the  special  function  of  forming  the  connective  tissues  and  blood. 
AYhether,  however,  this  is  actually  so,  must  be  i-egarded  as  at  present  undecided. 
However  produced,  the  appearance  of  a  middle  layer  causes  the  originally  bilaminar 
blastoderm  to  be  trilaminar,  and  its  three  layers  have  received  the  names  of  ectoderm, 
mesoderm,  and  entoderm,  or  epiblast,  mesoblast,  and  hy}ioljlast. 

The  gastrula  condition  of  the  vertebrate  ovum. — It  Avill  be  observed  that  in  the 
mammal  the  two  primary  layers  of  the  blastolerm.  at  least  their  principal  pai-t.  are  formed  by 
a  separation  into  two  strata  of  the  cells  of  the  inner  granular  mass  which  occupies  the 
interior  of  the  ovum  after  segmentation.  The  bilaminar  condition  may  therefore  be  .said  to 
result  from  a  process  of   deiamination  in  an  originally  simjile  mass  or  stratum.    But  in 


22  GASTEULATIO^s^    OF    VERTEBRATES. 

Amphioxus  among'st  vertebrates,'  and  in  many  inverteljrates  with  holoblastlc  (alecithal)  ova, 
the  bilaminar  blastoderm  is  produced,  not  by  delamination,  but  by  the  invagination  of  one 
pole  of  an  orig-inally  simple  hollow  spherical  blastodermic  vesicle,  the  invaginated  portion 
becoming'  the  primitive  entoderm  and  the  remaining  part  of  the  wall  of  the  vesicle  forming 
the  primitive  ectoderm  (fig.  24),  This  condition,  which  was  discovered  by  Kowalewsky,  is 
known  as  the  gastrula  Htarje,  and  it  is  regarded  by  most  embiyologists,  following  Haeckel,  as 
typical  of  the  mode  of  formation  of  the  bilaminar  blastoderm  throughout  the  animal  kingdom. 
The  aperture  of  invagination  by  which  the  cavity  of  the  entoderm  communicates  for  a  time 
with  the  exterior  has  been  termed  the  Mastojxire  (Lankester). 

It  is  not  possible  in  this  account  of  the  embryology  of  the  mammal  (which  must  necessarily 
be  very  short)  to  examine  at  any  length  the  evidence  upon  which  the  opinion  rests  that  a 
gastrula  stage  can  be  shown  to  exist  at  an  early  stage  in  the  development  of  the  meroblastic 
ova  of  the  lower  vertebrata.  It  will  be  sufficient  for  the  present  purpose  to  state  that  in 
fishes,  reptiles,  and  birds,  the  ova  of  all  of  which  are  of  a  markedly  meroblastic  type,  that 
part  of  the  ovum  in  which  alone  segmentation  has  occurred,  and  in  which  active  development 
subsequently  proceeds,  produces  a  bilaminar  blastoderm  as  in  the  mammal  by  the  separation  off 
as  a  distinct  layer  of  a  lower  or  inner  stratum  of  cells  to  form  the  primitive  entoderm, 
whilst  the  remaining  cells  arrange  themselves  into  an  upper  or  outer  stratum,  the  primitive 
ectoderm. 2  At  one  part  of  the  circular  blastoderm  which  has  thus  been  formed  there  now 
occurs  a  crescentic  thickening  of  the  ectoderm,  on  the  surface  of  which  a  pit  or  depression 
becomes  formed  by  an  invagination  of  the  ectoderm.  This  pit  extends  inwards  until  it  abuts 
against  a  subjacent  entodermal  thickening,  and  it  may  even  penetrate  the  entoderm  and 
communicate  with  the  cavity  below  the  blastoderm  (which  afterwards  becomes  in  part  con- 
verted into  the  posterior  end  of  the  alimentary  canal).  The  invagiuation  in  question  has  been 
regarded  as  a  rudimentary  blastopore,  its  time  of  formation  having  become  shifted  to  a  later 
period,  and  the  entoderm  having  already  been  formed  by  delamination  altogether  independently 
of,  in  place  of  resulting  from,  the  invagination,  as  in  the  typical  mode  of  gastrula  formation. 

In  the  mammal  a  similar  invagination  of  the  ectoderm  also  occrus  at  the  posterior  extremity 
of  the  embryonic  area,  and  this  invagination  has  been  described  by  Heape  in  the  mole  as 
communicating  for  a  time  with  the  cavity  of  the  blastodermic  vesicle  (fig.  22,  JZj;),  which  sub- 

—   -      -.^  Fig.  25.  — Surface  view  op  ajt  embryonic   area  of  the  mole 

"^^  IN  WHICH     THE     MEDULLARY    GROOVE     HAS     BE&UN     TO     APPEAR 

IN    FRONT    OF    THE    PRIMITIVE    STREAK.        At    THE    JUNCTION    OF 
THE    TWO    A    SMALL    APERTURE    IS    SEEN  :      THIS    IS    THE    DORSAL 
/  OPENING    OF    THE    OBLIQUE    NEURENTERIC    CANAL.       (Heape.) 

i  \        sequently  becomes  converted  in  part  into  the  alimentary  canal. 

^-  \        In  birds  and  reptiles  as  well  as  mammals  the  invagination  in 

f, "  ,'        question  soon  becomes  extended  forward  along  the  middle  line 

'(  of  the  blastoderm  as  a  linear  groove  (primitive  groove),  which 

t  indents  an  ectodermal  thickening  (primitive  streak),   and  if 

\  the    posterior    invagination    represents    a    blastopore,    this 

V  groove  must  be  looked  upon  as  an  extension  of  such  blas- 

topore,   a    view  which  derives   support   from  the  fact   that 
^'  there  appears  to  be  a  tendency  for  the  primitive  groove,  at 

\  ^  least  its  anterior  end,  to  penetrate  to  the  entoderm,  and  thus 

to  form  here  also  a  canal  of  communication  between  the  cavity 
below  the  entoderm  and  the  exterior.     Such  a  canal  is  desig- 
nated "  neurenteric,"  because  the  anterior  end  of  the  primitive  streak  and  groove  becomes 
eventually  enclosed  by  the  neural  tube,  and  the  canal  then  effects  a  (temporary)  communica- 
tion between  the  neural  tube  and  the  enteric  canal. 

Another  important  point  of  resemblance  between  this  invagination  and  the  blastopore  of 
the  typical  gastrula  is  the  fact  that  the  middle  layer  of  the  trilaminar  blastoderm  begins  to 
develope  from  the  margins  of  the  invagination.  But  in  this  respect  again  there  is  a  differ- 
ence, for  whereas  in  the  simplest  and  most  typical  forms,  such  as  Sagitta  amongst  invertebrates, 
and  Amphioxus  amongst  vertebrates,  the  middle  layer  (mesoblast)  originates  as  a  pair  of  hollow 
protrusions  of  the  primitive  entoderm  (coelom-invaginations  of  Hertwig,  figs.  28,  29)  ;  in 
mammals  and  birds  it  makes  its  first  appearance  in  the  form  of  solid  outgrowths  from  the 
primitive  streak.-* 

Other  views  concerning'  the  gastrulation  of  vertebrates. — Kupffer  regards  the   part 

'  Also,  according  to  Hofmann,  to  some  extent  in  elasmobrancli  fislies. 

2  No  layer  corresponding  with  Rauber's  layer  of  the  mammal  is  known  to  exist  in  lower  vertebrates, 
unless  that  layer  is  to  be  regarded  as  the  homologue  of  the  external  (corneous)  stratum  of  the  epiblast, 
which  is  found  at  a  later  stage  in  fishes  and  amphibia. 

2  Riickert  has  described  an  imperfect  form  of  ccelom-invagination  in  elasmobranch  fishes,  and 
Hertwig  in  amphibia. 


HISTOKY    OF    BLASTODERM.  iic 

•which  has  hten  above  alluded  to  as  iiivaginated  ectoderm  (primitive  groove)  as  the  homologuo 
of  part  of  the  entoderm  of  more  typical  forms.  If  this  view  Ije  correct,  many  of  tiiedilKculties 
in  the  way  of  re^ardintr  tlie  aperture  of  the  inva^rination  as  the  blastopore,  and  in  explaining,' 
the  differences  in  the  mode  of  origin  of  the  mesoblast  are  removed  ;  but,  on  the  other  hand, 
other  ditliculties  are  introduced,  and  the  subject  is  left  by  no  means  clear. 

Another  view,  which  wjxs  formerly  extensively  held,  regards  the  blastopore  of  the  nierolilastic 
vertebrate  ovum  as  bounded  by  the  thickened  edge  of  the  bilaminar  blastoderm  (Haeckel). 
According-  to  this  view,  the  cavity  of  the  gastrula  is  entirely  filled  up  by  a  mass  of 
unsi'gmcnted  or  but  partially  segmented  yolk,  which  also  projects  for  a  considerable  distance 
through  the  blastopore,  forming  in  fact  the  great  mass  of  the  ovum.  The  primitive  groove 
is  regarded  as  a  linear  prolongation  of  this  thickened  edge  of  the  blastoderm  towards  the 
centre  of  the  blastoderm  (Balfour),  so  that  the  embryo,  which  developes  in  front  of  the 
primitive  str(.ak,  thus  comes  to  have  a  i)seudo-central  position  in  the  blastoderm  instead 
of  developing  altogether  from  its  margin  as  in  the  lower  vertebrata  and  in  inverte- 
brates. In  conformity  with  this  idea,  it  may  be  noted  that  at  the  thickened  rim  of  the 
blastoderm  of  these  meroblastic  ova,  the  two  primary  Layers  are  continuous  with  one  another 
as  in  the  primitive  streak.  In  elasmobranchs  an  intermediate  condition  is  observed,  viz..  a 
short  groove,  the  margins  of  which  are  freely  continuous  with  the  margin  of  the  blastoderm. 
If.  as  His  and  others  have  described  (ji-ide  Infra),  the  mesoblast  is  in  part  (vascular 
and  connective  tissue  part)  derived  from  the  thickened  rim  of  the  blastoderm,  this  would 
fm-nish  another  point  of  resemblance  between  the  primitive  streak  and  that  margin. 

Ed.  V.  Beneden  has  promulgated  an  entirely  different  opinion  as  to  the  mammalian  blasto- 
pore from  those  above  described.  He  regards  the  condition  of  the  ovum,  after  the  completion 
of  segmentation  and  before  the  formation  of  a  blastodermic  vesicle,  as  representing  the 
gastrula  stage,  and  looks  upon  the  point  where  the  granular  inner  mass  of  cells  comes  to  the 
surface  between  the  clear  cells  which  form  the  outer  investment  as  the  blastopore  (fig.  14,  «). 
In  confoi-mity  with  this  view  he  considers  the  layer  of  clear  cells  to  represent  the  whole  of  the 
primitive  ectoderm,  and  the  granular  inner  mass  the  primitive  entoderm.  But  since  all  the 
more  recent  observations  upon  early  mammalian  ova  agree  in  affirming  the  formation  of 
the  three  blastodermic  layers  from  the  granular  inner  mass,  and  that  Eauber's  laj-er  either 
takes  no  pai-t  at  all,  or  only  a  subordinate  part  in  the  formation  of  the  ectoderm  of  the 
embryonic  area,  v.  Beneden's  view,  in  spite  of  the  superficial  resemblance  of  the  ovum  at 
this  stage  to  certain  gastrula  forms,  has  not  met  with  general  acceptance  from  embryologists. 

Inversion  of  the  blastodermic  layers  in  some  mammals.  —  In  the  guinea-pig 
(Bischoff),  rat  and  mouse  (Fraser,  Selenka),  and  in  some  other  rodents,  an  inversion  of  the 
usual  position  of  the  blastodermic  layers  is  found  to  occur,  the  epiblast  being  innermost, 
the  hypoblast  outermost.  The  foundation  of  this  inversion  is  laid  early  by  a  process  of 
invagination  and  formation  of  a  central  cavity  in  the  mass  of  entomeres,  so  that  when  the 
blastoderm  is  differentiated,  the  innermost  cells  which  are  next  the  (secondary)  cavity  thus 
formed  become  the  epiblast,  and  the  outermost  the  hypoblast,  the  mesoblast  subsequently 
forming  between  the  two  by  proliferation  of  epiblast  at  the  primitive  groove,  as  in  other 
mammals.  (For  details  as  to  this  process  of  invagination,  the  student  is  referred  to  the  papers 
by  Selenka.) 

Historical. — The  existence  of  several  laminse  in  the  germinal  substance  of  the  eg^  was 
first  suggested  by  C.  F.  Wolff  in  his  celebrated  work  Theoria  Generationis.  published  in  1759, 
and  in  his  later  Memoir  On  the  DevdojJinent  of  the  Intestine,  &:st  published  in  Nov.  Comment. 
Acad.  Petropol.  in  1767  and  republished  in  German  by  J.  F.  Meckel  in  1812.  It  is,  however,  to 
the  researches  of  Pander,  conducted  under  the  dii-ection  of  Dollinger  of  "Wiii-zburg,  and  pub- 
lished in  1817,  and  those  of  v.  Baer  (182G-lSo7),  tliat  we  owe  the  first  consistent  attempt  to 
connect  the  development  of  the  several  organs  and  systems  of  the  embryo  with  the  different 
constituent  parts  or  layers  of  the  blastodeim.  Pander  recognised  a  trilaminar  structure  of  the 
blastoderm  and  distinguished  the  three  layers  composing  it,  in  their  order  from  above  down- 
wards, or  from  without  inwards  in  the  eg'^,  as  the  serous,  vascular,  and  mucous  layers. 

In  lS.oO-."j-l  a  further  important  advance  was  made  in  the  knowledge  of  the  constitution  of 
the  blastodennic  layers,  by  the  discovery  by  Remak  that  the  greater  part  of  the  middle  layer 
soon  after  its  formation  comes  to  be  divided  into  two  laminaj,  separated  by  a  space  which 
coiTesponds  to  the  perivisceral  cavity  {calom') — a  fact  which  had  been  partially  stated  by  von 
Baer.  So  marked  a  division  of  the  middle  layer  and  distinction  of  the  parts  which  are 
afterwards  developed  from  its  two  laminae,  has  seemed  sufficient  to  some  authors  to  warrant 
the  recognition  of  four  distinct  layers  in  the  blastoderm  ;  but  it  will  be  found  on  the  whole 
more  convenient  to  consider  the  fundamental  layers  as  only  three,  to  which,  following  the 
nomenclature  of  Foster  and  Balfour,  the  designations  of  epiblast,  mesoblast,  and  hypoblast  are 
applied,  terms  which  are  synonymous  with  those  of  ectoderm,  mesoderm,  and  entoderm, 
employed  by  many  authors. 

The  terms  ectoderm  and  entoderm  were  first  applied  to  the  two  fundamental  layers,  shown 
by  Huxley  in  1849  to  constitute  the  whole  body  of  coelenterates.  and  which  were  correctly 
regarded  by  him  as  homologous  with  the  two  layers  of  the  bilaminar  blastoderm,  to  which  w"» 


24 


CHARACTERS    OF    BLASTODERMIC    LAYERS. 


have  apiDlied  the  terms  primitive  ectoderm  and  primitive  entoderm.  Since  the  middle  layer 
is  developed  from  one  or  both  of  these  primitive  layers  their  permanent  representatives  are 
morphologically  different,  having-  lost  the  elements  -w^hich  go  to  form  the  middle  layer,  and  it 
is  therefore  convenient  to  accentuate  this  distinction  by  the  adoption  of  different  terms  to 
represent  the  permanent  layers. 

The  generalisation  that  the  formation  of  a  bilaminar  blastoderm  is  typically  produced  by 
the  invagination  of  a  hoUov^r  spherical  unilaminar  blastodermic  vesicle  is  due  to  Haeckel,  and 
was  based  largely  upon  the  important  researches  of  Kowalevsky,  especially  those  on  Sagitta 
and  Ampjhioxus.  The  process  of  delamination  which  in  some  animals  produces  the  two 
primary  layers  was  originally  regarded  by  Ray  Lankester  as  the  typical  mode  of  formation, 
but  is  now  generally  admitted  to  be  a  secondary  modification.  Finally,  it  has  been  shoAATi 
(Balfour,  Lankester,  R.  and  0.  Hertwig),  as  is  set  forth  below,  that  the  coelom  or  body  cavity 
is  typicatty  developed,  not  by  a  process  of  splitting  of  the  mesoblast  (although  in  some 
animals  this  may  occur  as  a  secondary  modification),  but  as  hollow  protrusions  from  the 
primitive  alimentary  cavity,  the  cells  which  bound  these  protrusions  forming  the  mesoblast. 
Thus  from  an  originally  single  blastodermic  layer  by  successive  processes  of  invagiuation 
or  folding,  the  three  permanent  laminae  are  ultimately  produced. 

Such  folds  may  be  regarded  as  formed  mechanically  by  local  hypertrophic  multiplication 
of  the  cells  of  the  laminEe,  an  increased  surface  being  thus  found  for  the  increased  number  of 
cells.  In  analogous  manner  the  folds  which  accompany  the  formation  and  separation  of  the 
body  and  the  development  of  the  several  organs,  e.g.,  the  nervous  system,  alimentary  canal, 
amnion,  may  also  be  regarded  as  resulting  mechanically  from  cell-multiplication.  This 
mechanical  theory  of  development  was  first  enunciated  by  Pander,  and  has  of  late  years  been 
applied  extensively  by  several  embryologists,  notably  by  His  QEntwickl.  d.  Iluhnchens,  1868, 
and  Un-wre  Korperform.  1874),  and  Rauber. 

Characters  of  the  blastodermic  layers. — The  three  layers  of  the  blastodenn 
show  from  the  first  distinctive  characters  (fig.  2G).  The  outer  layer,  or  epiblast,  is 
epithelial  in  nature  and  consists  of  somewhat  irregularly  columnar  cells  closely  set 


Fig.  26. — Transverse  section  through  the  front  end  of  the  prijiitive  streak  ani> 

BLASTODERM  OP  THE  CHICK.     (From  Balfour. ) 

fr,  primitive  groove  ;  m,  mesoblast ;  cp,  epiblast ;  liy,  hypoblast. 

side  by  side,  forming  a  single  stratum  for  the  most  part,  except  near  the  middle 
line,  and  becoming  thinner  and  flatter  towards  the  margins  of  the  embryonic  area. 

The  inner  layer  or  hypoblast  is  also  epithelial,  but  the  cells  are  at  first  all 
flattened,  and  appear  therefore  quite  thin  and  linear  in  sections  of  the  blastoderm. 
At  a  later  stage,  the  hypoblast  cells  become  markedly  columnar  and  enlarged,  so 
that  they  considerably  exceed  the  epiblast  cells  in  size. 

The  middle  layer,  or  mesoblast,^  which  diff"ers,  as  we  have  seen,  in  its  mode  of 
arigin,  being  formed  secondarily  from  one  or  both  of  the  primary  layers,  also  diflTers 
Irom  them  entirely  in  its  appearance  and  structure.  Instead  of  consisting  of  cells 
closely  joined  together  into  a  continuous  membrane  after  the  manner  of  an 
epithelium,  the  mesoblast  is  at  first  composed  of  cells  which  are  not  thus  closely 
arranged,  but  have,  on  the  contrary,  a  considerable  amount  of  intercellular  fluid 
between  them.  They  are  most  irregular  m  shape,  and  are  often  branched  and 
united  with  one  another,  so  that  much  of  the  mesoblast  early  resembles  an 
embryonic  connective  tissue. 

i  The  mesoblast  was  by  von  Baer  described  as  being  formed  of  two  layers,  one  derived  by  splitting 
from  each  of  the  two  primary  layers,  and  although  this  mode  of  origin  can  no  longer  be  maintamed, 
the  two  strata  of  mesoblast  separated  bv  the  pleuroperitoneal  cavity  or  ccelom  are  still  by  some 
embryologists  considered  to  be  distinct  layers  of  the  blastoderm,  which  is  thus  described  as  quadri- 
laminar  (see  p.  26). 


I'AKAliLAST    THEOKY    OF    HIS. 


25 


S 


b^ 


Before  proceedinir  tu  desfiibe  the  commencinjr  tk-vclojiiuent  of  the  embryo  it  will 
be  instructive  to  enmnerate  the  jiarts  which  are  forniL*d  res])L'ctively  from  tlie  three 
blastodermic  layers.     The  following;  is  the  relation  giveu  in  tabular  form  : — 

'       The  whole  of  the  nervous  system,  including  not  only  the  central  oryans  (brain  and 
spinal  cord),  but  also  the  peripheral  nerves  and  sympatlictic. 
Thf  epithelial  structures  of  the  organs  of  special  sense. 
The  epidermis  and  its  appendajres.  includin^r  the  hair  and  nails. 
The  epithelium  of  all  the  ;,'lands  openin;r  upon  the  surface  of  the  skin,  including 
the  mammary-  irlands.  the  sweat  ;rlands.  and  the  sebaceous  glands. 
I        The  muscular  fibres  of  the  sweat  irlands. 

I  The  epithelium  of  the  mouth  (except  that  covering,'  the  ton^'ue  and  the  adjacent 
I  posterior  part  of  the  floor  of  the  mouth,  which  is  derived  from  hypoblast;,  and  that  of 
I  the  trlands  openin<r  into  it.     The  hypophysis  cerebri.     The  enamel  of  the  teeth. 

The  epithelium  of  the  nasal  passajres,  of  the  adjacent  upp^r  part  of  the  pharynx, 
and  of  all  the  cavities  and  glands  opening  into  the  nasal  passages. 
^    The  epithelium  of  the  anus  and  immediately  adjacent  pai-t  of  the  rectum. 
[       The  epithelium  of  the  vagina  and  of  the  urethra. 

The  urinary  and  generative  organs  (except  the  epithelium  of  the  urinary  bladder 
and  urethra). 

All  the  voluntary  and  involuntaiy  muscles  of  the  body  (except  the  muscular  fibres 
of  the  sweat  glands). 

The  whole  of  the  vascular  and  lymphatic  system,  including  the  serous  membranes 
and  spleen. 

The  skeleton  and  all  the  connective  tissue  structures  of  the  body. 

The  epithelium  of  the  alimentaiy  canal  from  the  back  of  the  mouth  almost  to   the 
anu<.  and  that  of  all  the  glands  which  open  into  this  part  of  the  alimentary  tube. 
The  epithelium  of  the  Eustachian  tube  and  tympanum. 
The  epithelium  of  the  bronchial  tubes  and  air  sacs  of  the  lungs. 
The  epithelium  lining  the  vesicles  of  the  thjToid  body. 
The  epithelial  nests  of  the  thymus. 
The  epithelium  of  the  urinary  bladder. 


PAHAELAST    THEORY  OF   HIS.      MESENCHYME    THEORY   OF   HERTWIG. 


^'^-^J 


The  observations  of  His  upon  the  development  of  the  blood  and  connective  tissues 
in  the  bird  led  him  to  regard  these  tissues  as  originating,  not  from  the  mesoblast  which 
in  the  chick   ^tows   out  from   the   sides   of   the   primitive   groove,  but  from  cells  which, 


Fig.   27. — Vertical  section  THKoroH  thl  blastoderm  of  a  hex's  egg  taken  near  the 

PERiPHEKT.     (Strieker.) 

E,  epiUast  :  JI.  hypoblast,  passing  at  the  periphery'  into  an  undifferentiated  mass  of  yolk,  A, 
containing  large  cells  filled  with  yolk  granules  ;  M  (towards  the  centre  of  the  blastoderm),  mesoblast  ; 
M  ('nearer  the  periphery),  granular  cells,  apparently  derived  from  A,  and  lying  between  the  epiblast 
and  hypoblast. 


originating  either  in  the  yolk  or  in  the  thickened  rim  of  the  spreading  blastoderm,  wander  in 
centripetally  ';etween  the  primaiy  layers  and  fill  up  all  the  interstices  of  the  centrifu gaily- 
growing  true  mesoblast.  These  in-wandering  cells  being  derived,  not  lie  the  other  cells  of 
the  embryonic  area  from  the  more  active  primarily  differentiated  central  parts  of  the  blasto- 
derm, but  from  the  peripheral  non-embryonic  portion,  were  collectively  named  by  His 
jiara blast,  and  the  tissues  (blood  and  blood-vessels,  and  all  the  connective  tissues)  supposed  to 
be  fonned  from  them  were  termed  jMii-aJAa.^tic  (all  the  other  tissues  of  the  embryo  being 
termed,  in  contra-distinction.  archlhlaiitic'). 

Hiss   theory  was   enunciated  as  long  ago  as   1S68.  although   he  afterwards   introduced 
into  it  certain  modifications.    For  a  considerable  time  it  met  with  little  acceptance,  but  of 


26 


MESENCHYME    THEORY    OF    HERTWIG. 


late  years  it  has  olDtained,  in  a  somewhat  modified  fonn,  the  adherence  of  many  embiyologists, 
and  especially  of  R.  and  0.  Hertwig,  Kupffer,  Kollmann,  and  Waldeyer.  R.  and  0.  Hertwig- 
have  given  the  name  of  mesenchyme  to  the  embryonic  tissue  which  gives  rise  to  the  connective 
tissues  and  vascular  endothelium,  while  retaining  the  designation  of  middle  germinal  layer 
or  mesoderm  for  the  rest  of  the  mesoblast  ;  but  the  derivation  of  the  mesenchyme  is 
admitted  by  0.  Hertwig  to  be  directly  from  mesoblast,  from  which  it  differs  (1)  in  its 
structure,  consisting  of  loosely  arranged  wandering  cells,  as  distinguished  from  the 
epithelium-like  lamellse,  of  which  according  to  their  description  the  rest  of  the  mesoderm  is 
composed  ;  (2)  in  its  derivation,  arising  as  separate  cells  from  the  entoderm  instead  of  in  the 
form  of  a  coherent  layer  ;  and  (3)  in  its  further  development  and  destination,  giving  origin 
to  the  connective  tissues  and  blood-vessels,  and  jierhaps  to  the  plain  muscular  tissue,  whereas 
the  mesoderm  proper  gives  origin  to  the  skeletal  muscles  and  to  the  epithelium  of  the  serous 
cavities,  and  of  the  genital  and  urinary  organs.  They  describe  the  true  mesoderm  as 
consisting  of  two  epithelial  lamellcE,  which  form  distinct  layers  of  the  blastoderm,  so  that 
according  to  this  view  the  complete  blastoderm  Avould  consist  otfonr  layers  (epiblast,  outer 
or  somatic  mesoblast,  inner  or  splanchnic  mesoblast  and  hypoblast)  besides  the  mesenchyme  : 
to  which  must  be  added  a  median  strand  of  cells  set  aside  for  the  formation  of  the  notochord, 
and  derived  from  the  hypoblast. 

It  appears  evident  from  the  researches  of  Kowalevsky  in  Sagitta  and  Amphioxus  that  what 
is  to  be  regarded  as  the  typical  orig'in  of  the  mesoblast  in  Metazoa^  takes  the  form  of  a  pair  of 
diverticula  from  the  primitive  archenteric  cavity  (fig.  28,  la  ;  fig.  29,  I),  which  hollow 
diverticula  become  pinched  off  from  the  remainder  of  that  cavity,  and,  their  cavities  becoming 


Fig.    28. — Formation    of   mesoblastic    somites  ix  amphioxus,    shows   in    longitudinal    optical 

SECTION.      (Hatschek. ) 

la.  dorsal  view  of  an  embryo  in  which  the  mesoblast  is  beginning  to  form  as  two  longitudinal  folds 
of  the  hypoblast  which  are  becoming  subdivided  from  before  back  by  cou&trictions  into  separate 
somites.  I.,  the  same  viewed  in  profile,  showing  the  anterior  three  somites  of  one  side,  with  their 
cavities  in  free  communication  with  the  enteric  cavity.  The  neural  canal,  n.c,  is  continued 
posteriorly  by  a  neurenteric  canal  into  the  enteric  cavity;  ej),  epiblast;  hij,  hypoblast.  II.,  dorsal 
view  of  a  more  advanced  embryo.  The  somites  are  more  numerous  and  are  completely  separate.  In 
all  but  the  most  anterior  pair  the  communication  with  the  enteric  cavity  is  still  seen.  III.,  dorsal 
view  at  a  still  later  stage.  The  somite  cavities  are  now  completely  closed.  The  cellular  rod,  ch,  shown 
running  along  the  middle  of  the  embryo  is  the  notochord. 

compressed  laterally,  are  converted  into  the  coelom  or  body-cavity  (serous  cavities  of 
vertebrata),  the  two  walls  of  this  cavity  on  either  side  forming  respectively  the  inner  and 
outer  mesodermic  layers  of  R.  and  0.  Hertwig.  In  Sagitta.  the  diverticula  occur  in  the 
neighbourhood  of  the  blastopore,  which  is  also  the  typical  seat  of  origin  of  the  mesoblast,  but 
in  Amphioxus  they  are  formed  by  longitudinal  folds  of  the  wall  of  the  archenteric  cavity, 
which  grow  from  before  backwards,  and  become  separated  up  into  segments  in  theii-  progress. 
In   Elasmobranchs    the  myotomes   or   protovertebraj  are  at  an  early  period  hollow,  and  the 

1  Except  the  Coelenterata  which  have  only  the  two  primary  layers. 


RECENT    LITERATURE    OF    BLASTODEIIM. 


27 


cavities  communicate  witli  the  ca'loni.  In  most  vertebrates  above  Ampliioxus  the  mesoblastic 
outt,'-ro\vths  are  from  the  first  solid,  not  noUow  (althougli  a  split  may  early,  and  does 
eventually  in  any  case,  occur  in  them,  the  ccelom  being  thu.s  i)roduced;,  nor  do  they  originate 
so  distinctly  from  tiie  entoderm,  but  arise  rather  at  the  junction  of  this  with  ecto'lerm  at  the 
margin  of  the  blastopore,  and  in  the  higher  forms,  especially  mammals,  may  even  be  largely 
derived  from  ectoderm.  It  is  nevertheless  for  many  reasons  probable  that  the  origin  in  a  pair 
of  hollow  diverticula,  as  above  described,  is  to  be  looked  upon  as  the  typical  one,  and  that  as 


Fig.  29. — Skctions  across  ax  amphiuxvs   embryo   of    about  the  stages   shown  in  fig.  28,  I.  to 

III.     (Hatscbek.) 

71. y.,  neural  groove  ;  n.c,  neural  canal ;  ch,  rudiment  of  notochord;  mcs.  som.,  mesoblastic  somite. 
In  I.,  its  cavity  is  in  free  communication  w-ith  the  alimentary  cavity  ;  C2),  epiblast ;  hi/,  hypoblast  ;  al, 
alimentary  cavity.  In  III.  the  cavity  of  the  somite  has  extended  on  either  side  of  tlie  alimentary 
canal  and  forms  a  ccelom,  or  body  cavity  (cce). 

a  soUd  outgrowth,  subsequently  becoming  split  or  hollow,  as  a  secondary  modification.'  It  is 
questionable,  however,  whether  there  is  so  considerable  a  difference  between  the  external  and 
internal  portions  of  the  wall  of  the  diverticulum  that  these  two  plates  of  mesoblast  shoidd  be 
regarded  each  one  as  of  equal  morphological  importance  with  the  epi-  and  hypoblast. 

The  mesenchyme  elements  are  not  essentially  different  in  their  origin  from  the  rest  of 
the  mesoblast.-  In  forms  which  are  regarded  as  most  typical,  such  as  Sagitta  and 
Amphiox;us,  they  are  not  distinct  in  origin  from  that  layer.  In  the  simpler  forms  amongst 
the  Craniata,  as  Cyclostomata  and  Amphibia,  no  origin  distinct  from  the  rest  of  the 
mesoblast  has  been  described  for  these  elements,  nor  has  it  been  seen  in  mammals,  in  which, 
indeed,  it  is  difficult  to  conceive  an  independent  source  for  them.  In  Elasmobranchs,  at  a 
time  when  the  cavity  of  each  myotome  or  protovertebra  is  still  in  open  communication  with 
the  coelom.  a  large  number  of  amceboid  cells  separate  from  the  adjacent  mesoblast  and  take 
up  a  position  between  the  myotome  and  the  notochord.  These  form  the  so-called  sclerotome. 
which  gives  origin  to  the  axial  skeleton.  Other  similar  masses  of  cells  are  given  off  from 
other  parts  of  the  mesoblast  (splanchnopleural.  somatopleural  and  integumental).  It  is  only 
in  the  highly  modified  meroblastic  ova  that  appeai-ances  have  been  noted  which  have  seemed 
to  justify  the  ascribing  a  peripheral  origin  to  the  parablastic  elements.  But  the  evidence 
which  has  been  hitherto  adduced  in  favour  of  this  view  cannot  be  regarded,  as  sufficient  to 
justify  its  unconditional  adoption,  and  it  must  be  regardei  as  equally  open  to  consideration 
whether  the  derivation  from  that  part  of  the  blastoderm  which  is  most  closely  connected 
with  the  source  of  nutriment,  viz.,  the  yolk,  of  those  elements  which  are  to  form  the  blood 
and  blood-vessels,  and  otherwise  to  minister  to  the  nutrition  of  the  early  embryo,  is  not  to  be 
explained  by  the  modified  physiological  conditions  of  these  telolecithal  ova. 


'  R.  and  0.  Hertwig,    "Die  Ccelomtheorie,"  Jena,  ISSl. 

-  Cf.  Balfour,    "Comparative  Embryology,"  vol.  ii.,  pp.  296,  297. 


28  EECENT    LITEPtATUEE    OF    BLASTODERM. 


RECENT    LITERATURE. 1 

Balfour,  F.  MC.,  On  the  structure  and  homologies  of  the  germinal  layers  of  the  emh-ijo.  Quart. 
Journ.  Microsc.  Sc,  vol.  xx.,  1880. 

Balfour,  F.  M.  and  F.  Deigliton,  A  reneioed  study  of  the  germinal  layers  of  the  chicle 
Quart.  Journ.  Microsc.  Scienc,  vol.  xxii..  1882. 

Beneden,  Ed.  van,  Becherches  sur  Vembryologie  des  mammiferes.  La  formation  des  feuillets 
chez  le  lapin.  Archiv.  de  biologie,  t.  i.  1880  ;  Sur  I'dvolution  de  la  ligne  primitive,  la  formation 
de  la  notocorde  et  du  caned  cordal  chez  les  mammiferes  {lapin  et  murin).  Bulletin  de  racademie 
royale  de  Belgique.     Ann.  v.,  s^r.  iii.,  t.  xii.,  1886. 

Beneden,  Ed.  van  und  Ch.  Julin,  Oiservations  sur  la  maturation,  la  fecondation  et  la 
segmentation  de  I'cenf  chez  les  cheiropiteres.     Arch,  de  biol. ,  t.  i.,  1880. 

Bonnet,  K.,  Ueher  den  Primitivstreifen  und  die  Chorda  der  Wiederkauer.  Sitzungsber.  d. 
Gesellscliaft  f .  Morphologie  u.  Physiologic  zu  Mtinchen,  18S6  ;  Beitrdgezur  Emhryologieder  Wicderkduer. 
Arch,  fur  Anat.,  1884,  1889. 

Boveri,  T.,  Ueber  Differenzirung  der  Zellkerne  wdhrend  der  Furchung  des  Eies  von  Ascaria 
■fuegalocephala.     Anat.  Anzeig.,  1887. 

Blitsclili,    O.,  Beitrdge  zur  Gastndatheorie.     Morpholog.  Jahrbuch,  Bd.  ix.,  H.  3,  1884. 

Caldwell,  "W.  H.,  The  embryology  vf  Monotremata  and  Marsupialia.  Philosophical  Trans., 
vol.  clxxviii.,  1888. 

Durham,  H.,  Note  on  the  presence  of  a  neurenteric  canal  in  liana.  Quarterly  Journal  of 
Microscopical  Science,  N.  Ser.,  vol.  26,  1886. 

Duval,  M.,  Be  la  formation  du  ilastoderme  dans  Vceaf  d'oiseau.  Annales  des  sciences 
naturelles,  6  ser.  t.  xviii.,  No.  1—3,  1884. 

Fleiscliniann,  A.,  Zur  Entwickclungsgeschichte  der  Raubthiere.     Biolog.    Central bl.,   Bd.    vii., 

1887. 

Gasser,  Der  Parablast  u.  der  Keimwall  der  Vogelkeimscheibe.  Sitzungsber.  d.  naturw.  Gesellsch. 
EU  ^[arburg,  1883. 

G-erlach,  L.,  Ueber  die  entoclermale  Entstehung  der  Chorda.     Biol.  Centralbl.,  1881. 

Giacomini,  C,  Sul  canale  neurenterico  et  sul  canale  anale  nclle  vesicole  blastodermiche  di 
■coniglio.      Giorn.  della  r.  accademia  di  medic,  di  Torino,  No.  4,  5,  1888. 

Haddon,  A,,  Note  on  the  blastodermic  vesicle  of  mammals.  Proceedings  of  the  Royal  Dublin 
Society,  N.  Ser.,  vol.  iv.,  1385. 

Haeckel,  Bie  Gastrceatheorie.  Jena  Zeitschrift,  Bd.  viii.  ;  Nachtrdge  zur  Gastrceathcorie. 
Ibid.,  Bd.  xi.  ;   Ur sprung  u.  Entivickl.  d.  thierischen  Geiueben.     Ibid.,  Bd.  xi. 

Hatschek,  B.,  Studien  iiber  Entwicklung  des  Amphioxus.  Arbeiten  a.  d.  zool.  Instit.  zu  "Wien, 
Bd,  iv. ,  1881  ;   Ueber  die  Entioicklung  des  Amphioxus.     Biolog.  Centralbl.,  Bd.  vi.,  1887. 

Heape,  W.,  The  development  of  the  mole  (Talpa  europcea).  The  formation  of  the  germinal 
layers,  and  early  development  of  the  medullary  groove  and  notochord.  Quart.  Journ.  of  IVticrosc. 
Sc,  N.  S.,  xci.,  1883.  Also  in  Studies  from  the  Morphological  Laboratory  in  the  University  of 
Cambridge,  vol.  iii. 

Hensen,  Ueber  die  Ableitung  der  Vmkehr  der  Keimbldtter  des  Meersehiveinch^ns.  Verhand- 
lungen  des  physiologisclien  Vereins  in  Kiel,  1881. 

IIert\s*g-,  O.,  Bie  Bntwicklung  des  mittleren  Keimblattes  der  Wirbelthiere.  Jena  Zeitschrift  fiir 
Naturw.,  Bd.  xvi.,  1882. 

Hertwig,  O.  und  E.,  Die  Cblomtheorie.  Versuch  einer  Erkldrung  des  mittleren  Keimblattes. 
Jena,  1881 ;  Studien  zur  Bldttertheorie.     Jena,  1883. 

His,  "W.,  Der  Keimwall  des  Hiihnereies  u.  d.  Entstehung  der  parablastischen  Zdlen.  Zeitsch.  f. 
Anat.  u.  Physiol.,  Anat.  Abth.,  1876  ;  Die  Lehre  vom  Bindesubstanzkeim  (Parablast),  Ruckblick  nebst 
■critischer  Besprechung  einiger  neuerer  entwickiungsgeschichtlicher  Arbeiten.  Archiv  fiir  Anat.  u. 
Pijy.'^inl.,  Anatom.  Ab'tlieil.,  1882. 

Hoffmann,  C.  K.,  Die  Bildung  des  Mesoderms,  die  Anlage  der  Chorda  dorsalis  und  die 
Entivickelung  des  Canalis  neurentericus  bei  Vogelembryonen.     Amsterdam,  1883. 

Hubrecht,  A.  A.  "W.,  Bie  erste  Anlage  des  IJypoblastcs  bei  den  Sdugethieren.  Eine  Erwiderung 
■an  Herrn  Prof.  Bd.  van  Beneden.     Anatomischer  Anzeiger.,  iii.  Jahrg.,  No.  30,  1888. 

Johnson,  Alice,  On  the  fate  of  the  blastopore  and  the  presence  of  a  primitive  streak  in  the 
NeiDt  [Triton  cristatus).     Quart.  Journ.  of  Microsc.  Science,  N.  S.,  No.  xcvi.,  1884. 

Keibel,  F.,  Van  Beneden's  Blastoporus  und  die  Rauher'sche  Deckschicht.  Anatomisch.  Anz., 
1887  ;  Die  Enticickclungsvorgdnge  am  hinteren  Ende  des  Meerschiceinchencmbryos.  Arch.  f.  Anat. 
u.  Physiol.,  Anat.  Abth.,  H.  5  u.  6,  1888. 

KoUer,  C,  Untersuchungen  iiber  die  Bldtterbildung  im  Hilhnerkeim.  Arch.  f.  mikr.  Anat., 
1881  ;  Beitrdr/e  zur  Kenntniss  des  Hiihnerheims  im  Beginne  der  Bebriitung.  Wiener  Sitzungsber., 
Bd.  Ixxx.,  1881. 

Kolliker,  Die  Entioicklung  der  Keimbldtter  des  Kaninchens.  Wurzburg  Festschrift.  Leipzig, 
1882;  Die  embryonalen  Keimbldtter  u.  d.  Gewebe.  Zeitschr.  f.  wiss.  Zool.  xl.,  1884;  Ueber  die 
Nichtexistenz  eines  embryonalen  Bindegewebskeims  [Parablasts).  Sitzungsber.  d.  phys.  medic.  Ges. 
zu  Wurzburg,  1884. 

1  For  an  extended  bibliography  of  the  maturation,  fertilization  and  segmentation  of  the  ovum,  see 
Sobotta,  "  Die  Furchung  des  Wirbelthiereies  "  in  Ergebnisse  der  Anatomie  u.  Entwickelungsgeschichte 
von  Merkel  u.  Bonnet,  1896. 


RECENT    LITEKATUltE    OF    BLASTODKll.M.  29 

Kollmann,  J.,  Ikr  Mtsuhhttt  und  die  Entwirkelung  der  Oewehe  bei  WirbeUhieren.  Biolo;^. 
Ceiitnilbhitt,  UJ.  iii.,  1SS4;  Jkr  Unndundst  u.  d.  Uriprutuj  d.  Slutz»ub»tanz.  Arch.  f.  Aii.it.  ii. 
rhysiol..  Aiiat.  Abtli.  1884. 

Kowalevsky,  J-Jntirickhtnf/sricsc/iir/itr  dir  f^uijillu.  St.  rettTMburg  Memoir.s,  xvi  ,  1S71  ; 
J:'ntwirk-linii//!;iiscli.  d.  Amji/iio.fiu,  dr.  Arcli.  f.  inikr.  Aiiat.,  IM.  .xiii.,  1877  ;  lebcr  die  erstfit 
Eidwickiluwi.ii'rocissf  dir  Knuchcnfisrhe.     Zeitschr.   f.    wisseiiscbiift.   Zooloxie,  H<1.  xliii  ,  H.  .'i,  188G. 

Kupflfer.  C,  l>iis  Ei  von  Arn'oihi  arrali.i  und  die  verincintliilii-  Umkihr  der  Kehiihtiitlir  an 
demsclbtii.  .MiiiK-hener  Sitzung.sbericlite,  II.  5,  1882  ;  Die  Gantrulalion  an  den  menblnstischin 
Eiern  dtr  U'irbdthicrc  xind  die  Bci'teutun;/  des  Priinitirstni/.^.  Arcliiv  f.  Anat.  utid  Physiol.,  Aunt. 
Abtheil.,  18S2  ;  I'e.  d.  Can<dig  neurentiricus  der  Wirbclthicre.  Sitzungsberichte  der  (ie.sellscli.  f. 
Morpliol.  11.  Pliysiologie  zu  Jltincheii,  1887. 

Lankester,  On  (he  prim  it  ire  cclldai/era  of  the  embri/o,  dr.  Annals  and  Mag.  of  Nat.  Hist.,  xi., 
18":!  :  Xii'i.t  "II  I  inl>riii>l'"/ij  and  r/aii.^itication.     Quarterly  Journ.  of  Miir.  Science,  xvii.,  1877. 

Mitsukuri,  K.,  and  Ishikawa,  C.  On  the  formation  of  the  f/erminal  lai/crs  in  Chelonia. 
Journal  ot  tlie  College  of  Science,  Ini|ierial  University,  Japan,  vol.  i.  1888. 

Morg-an,  Sotes  on  thr  fate  of  the  amphibian  blastopore,  John  Hopkins  University  Circulars,  1880. 

Perenyi,  1.,  Die  Enticickl.  der  KeimbUitter  u.  d.  Chorda  in  neuer  Beleuchtuny,  Anat.  An'zeiyer,  4. 

Piatt,  Studie.f  on  the  2)rimitire  axial  segmentation  of  the  chick,  Bulletin  of  tiie  Zool.  Museum  at 
Harvard  Culk-e,  1889. 

Sabl.  C,  fiber  die  BUdung  des  Mesoderms.   Anatomis:^hcr  Anzeiger,  iii.  Juhrg.,  No.  23 — 25, 1888. 

Bauber,  Ucber  d.  Ursprung  des  Blutes  u.  der  Bindesubatanzen.  Sitzuugsb.  d.  Naturf.  Gesellsch. 
ru  Leipzig,  1877  ;  Die  Entwickelung  der  Gewebe  des  Sdugethierkorpers  und  die  histologischen  Systeme. 
Ber.  der  Naturf.  Ges.  zu  Leipzig,  1883. 

Bavn,  E.,  Uebcr  die  mesodermfreie  Stelle  in  der  Keimscheibe  des  Iliihneremhryo.  Archiv  f. 
Anatomic  u.  Physiologic,  Anatom.  Abth.,  1886. 

Repiachoff,  W.,  Bema-kungen  iiber  die  Keim,Udtter  der  Wirbelthiere.     Zool.  Anzeiger,  vi.,  1883. 

Romiti,  Gr.,  Sur  Vorigine  da  misoderme  et  ses  rapports  avec  le  vitellus.  Arch.  ital.  de  biologie, 
t.  ii.,  1SS2. 

Roux,  W.,  Beitrdge  zur  Entioickelungsmechanik  des  Embryo.  Ueber  die  kUnstliche  £f error- 
bringung  halbcr  Embryonen  durch  eine  der  beiden  ersten  Purchungslcugeln,  d-c.  Virchow's  Archiv, 
Bd.  cxiv.,  1888. 

Riickert,  Ueber  die  Gastrulaiion  der  Selachier.     Biologisches  Centralblatt,  Bd.  vi. 

Ryder,  The  inversion  of  the  blastodermic  layers  in  Hesperornys.  American  Naturalist, 
vol.  xxi. 

Schanz,  F.,  Das  Schicksal  des  Blastoporus  bei  den  Amphibien.  Jenaische  Zeitschr.  f.  Natur- 
wissensch.,  Bd.  xiv.,  1887. 

Selenka,  Emil,  Keimhldtter  und  Primitivorgane  der  Maus.  Wiesbaden,  1883  ;  Die  Bldtterum- 
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Arnnioten.     Biolog.  Centralbl.,  1887. 

Shipley,  A.  E. ,  On  the  formation  of  the  mesoblast,  and  the  persistence  of  the  blastopore  in  the 
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Jahrb.,  x. 

Spee,  P.,  Beitrag  zur  Entwirkelungsgeschichte  der  fruheren  Sto.dien  des  Meerschweinchens  bis  zur 
Vollendung  dei-  Keimblase.     Arch,  fiir  Anat.  u.  Physiol.,  Anat.  Abth.,  Heft.  i.  u.  ii.,  1883. 

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so 


EARLY    CHANGES    IN    THE   BLASTODERM. 


EARLY    CHANGES     IW     THE     BLASTODERM,    RESULTING     IN 
THE     FORMATION     OF     THE     EMBRYO. 

FIRST    APPEARANCE    OF    THE    EMBRYO;    FORMATION    OF    THE    NEURAL   GROOVE 
AND    MBSOBLASTIC    SOMITES. 

Henral  canal. — The  blastoderm  in  mammals,  as  we  have  seen,  eventually 
completely  encloses  the  cavity  of  the  ovum,  and  even  in  the  large  telolecithal  ovum 
of  the  bird,  it  gradually  extends  so  as  to  cover  a  large  part  of  the  yolk.  But  only 
a  small  portion  of  this  membrane  takes  part  in  the  formation  of  the  body  of  the 
•embryo,  that  portion  namely  which  lies  immediately  in  front  of  the  primitive  groove, 
and  it  is  here  that  what  may  be  regarded  as  the  first  trace  of  the  embryo  makes  its 
appearance — very  soon  after  the  outgrowth  of  mesoblast  from  the  sides  of  that 


Fig.  30. — Ejibrtonic  area,  with  outline  of  part  of  the  vascular,  area,   FROir  A  rabbit's 
OVUM  OF  SEVEN  DAYS.     "f.     (From  Kolliker.) 

ag,  embr3'omc  area  ;  ^r,  primitive  streak  and  groove  ;  rf,  medullary  groove. 

■groove — in  the  form  of  a  shallow  furrow  (fig.  30,  rf),  wide  behind  where  it 
embraces  the  anterior  end  of  the  primitive  streak,  and  at  first  narrow  in  front,  and 
.bounded  on  either  side  and  anteriorly  by  a  fold  of  epiblast ;  which  folds,  in  fact, 


Fig.   31. — Section  across  the  anterior  part  of  the  medullary  groove  op  an  early  ejibryo 

OF    THE    guinea    PIG.       (E.   A.   S.) 

c^5,  folds  of  epiblast  rising  tip  on  either  side  of  the  middle  line,  and  thus  bounding  the  medullary 
•groove  ;  mff,  middle  of  medullary  groove  ;  hy,  hypoblast,  -which  is  in  contact  with  the  medullary 
epiblast  at  the  middle  of  the  groove,  but  is  elsewhere  separated  from  it  by  mesoblast,  m,  which  has 
burrowed  forwards  between  the  two  primary  layers.  A  cleft  is  seen  in  the  mesoblast  on  either  side  ; 
this  is  the  commencement  of  the  anterior  part  of  the  ccelom. 


FOK.MATKi.N    oK    NEL'EAL    CANAL. 


■U 


ccrn-. 


Fig.    32.— Sections   showing   stages   in   the    conversion   of   the   ^lEDCLLAsr^  groove   into   the 

NEURAL    CANAL.       FrOM    THE    TAIL    END    OF    .VN    EMBRTO    OF    THK    CAT.        (,K.    A.    S. ) 

ep,   me,  hy,  epiblast,  mesoblast,  and  hypoblast;    ra.fj.,  medullary  groove;  »;.c.   (in  IV.),   neui-al 
canal  ;  ch,  notochord  ;  c<t,  ccelom  ;  am,  tail  fold  of  the  amnion. 


33  THE    NOTOCHORD. 

produce  the  groove  which  is  enclosed  between  them.  The  groove  which  is  thus 
early  formed  in  front  of,  but  not,  as  was  formerly  supposed,  in  continuity  with  the 
primitive  groove  (Dursy),  is  no  less  than  the  rudiment  of  the  whole  central  nervous 
system,  and  it  is  accordingly  known  as  the  neural  or  medullary  groove,  the  folds  which 
bound  it  being  termed  the  medullary  folds.  By  the  liirLe""that  Tfie  neural  groove  is 
formed,  the  mesoblast  has '  generally  extended  forwards  from  either  side  of  the 
primitive  streak,  burrowing  between  the  epi-  and  hypoblast,  and  as  the  folds  become 
developed,  this  mesoblast  fills  up  the  space  below  the  epiblast,  triangular  in  section, 
which  each  fold  encloses,  so  that  on  either  side  of  the  neural  groove  there  is  now  a 
longitudinal  thickening  of  mesoblast,  entirely  separated  from  its  fellow  of  the 
opposite  side  by  the  meeting  of  epi-  and  hypoblast  at  the  bottom  of  the  neural 
groove  (fig.  31),  and  gradually  thinning  off  laterally  into  what  is  known  as  the 

Fig.   33. — Middle  of   the   section  shown   in    fig.    31, 

MAGNIFIED   TO    SHOW    THE    EETAILS    OF    ITS    STRUCTURE. 
(E.    A.   S.) 

ep,  me,  hy,  m.g. .  as  above  ;  nch,  notocliordal  thickening 
of  median  hyi^oblast. 

lateral  plate  of  mesoblast.  These  two  longi- 
tudinal thickenings  of  mesoblast  give  origin  to 
most  of  the  muscular  and  skeletal  tissues  of  the 
body  ;  they  form  what  may  be  termed  the^ 
Tic^-  paraxial  as  distinguished  from  the  lateral  meso- 

blast. Somewhat  later  the  medullary  folds: 
become  bent  over  the  neural  groove,  and  meet  one  another  in  the  middle  line 
(fig.  32).  Here  they  blend  together,  and  the  groove  becomes  converted  into  a. 
canal — the  neural  canal.  Of  the  two  layers  of  epiblast  which  are  formed  from 
the  folds,  one  is  now  the  roof  of  the  canal,  the  other  is  the  epiblast  of  the  dorsal 
surface  of  the  embryo.  The  layers  are  at  first  in  contact  with  one  another,  but 
subsequently  mesoblast  passes  between  them  (forming  the  menibrana  reuniens 
superior  of  Remak).  The  closure  of  the  neural  groove  begins  in  the  posterior 
cephalic  region,  and  thence  extends  forwards  and  backwards. 

Nqtochord. — Running  along  the  bottom  of  the  neural  groove  there  may  soon 
be  seen,  when  the  blastoderm  is  viewed  fi-om  above,  a  linear  shading,  which  appears 
to  start  from  the  anterior  end  of  the  primitive  streak  and  passing  forwards  becomes 
gradually  lost  towards  the  anterior  end  of  the  neural  groove.  Transverse  sections 
across  the  latter  show  that  this  appearance  of  shading  is  due  to  a  longitudinal 
thickening  of  the  hypoblast  along  the  middle  line  (fig.  33)  ;  the  central  cells  of 
this  layer  becoming  enlarged  and  gradually  separating  themselves  off  to  form  a  rod- 
like column,  which  lies  between  epi-  and  hypoblast  just  below  the  neural  groove 
(fig.  32,  ch).  When  so  separated,  the  column  is  known  as  the  cliorda^  dorsalis,  or 
notochprd,  a  structure  which,  along  the  middle  line  of  the  early  em'Bryo,  replaces  the 
mesoblast,  and  which  is  at  first,  as  before  said,  continuous  with  the  united  epi-  and 
hypoblast  at  the  anterior  end  of  the  primitive  streak.  The  actual  separation  of  the 
notochordal  cells  from  the  hypoblast  occurs  first  a  little  behind  the  anterior  end  of 
neural  gToove,  and  progresses  backwards,  although  the  hypoblastic  thickening  occurs 
first  at  the  posterior  end  of  the  neural  groove  and  is  in  fact  directly  continuous  with 
the  united  column  of  epiblast  and  hypoblast  which  forms  the  anterior  end  of  the 
primitive  streak.  The  neurenteric  canal  passes  through  the  thickened  posterior 
extremity  of  the  notochord,  where  this  is  continuous  with  the  anterior  end  of  the 
primitive  streak  (see  fig.  34).  It  is  continued  in  mammals  a  short  distance  along  the 
notochord  as  a  canal  (prolonged  forwards  into  a  groove)  (fig.  di,  b),  which  has  been 


THE   NOTOCHOKD. 


38 


Fig.     34. — A    SERIES    OF    TRANSVERSE     SECTIONS     THKOCGH     THE     NECREXTERIC    AND    NOTOCHORDAL    CANAL 

OF  A  MOLE  EMBRYO.     (Heape.) 

The  embryo  wa.s  slightly  more  advanced  than  the  one  represented  in  Fig.  1:k  The  dorsal  opening 
is  shown  in  I.,  continued  into  the  primitive  groove  :  the  canal  passes  thence  tliiough  the  column  of 
cells  which  unites  the  epiblast  and  hypoblast  at  the  front  of  the  primitive  streak  (II. ),  into  the  noto- 
chordal  thickening  of  the  hypoblast  (III.),  along  which  it  extends  for  some  distance  (IV.),  and 
eventually  opens  ventrally  (V.)  into  a  median  groove,  which  is  formed  in  the  notochordal  thicktniiig 
(n.  ch.  c. ) 

ep,  me,  hj,  epiblast,  mesobiast,  hyi>oblast  ;  p. (jr.  (in  1.  and  II.),  j.rimitive  groove  ;  c,  neureuteric 
or  notochordal  canal ;  vi.gr.  (in  III.,  IV.,  and  V.),  meduUary  groove. 

VOL.  I.  „ 


34 


SEPARATION    OF    THE    EMBRYO. 


described  undei'  the  uame  of  the  nolocJiordal  ranaJ,  and  corresponds  with  the  hypo- 
blastic  invagination,  which  in  Amphioxns  (fig-.  20, 1.,  di)  also  forms  the  first  stage  in 
the  development  of  the  notochord. 

A  flattening  out  and  even  eventually  a  duplication  of  this  canal  occurs  in  the  chick  and  in 
various  mammals  at  a  somewhat  later  stage  than  that  given  in  fig.  34.  According  to  Spee, 
its  cleft-like  cavity  may  pass  laterally  into  the  commencing  mesoblastic  cleavage  (ccelom- 
invagination  .'). 

The  notochord  is  essentially  an  embryonic  strQcture  in  mammals,  although  it  does 
not  completely  disappear,  for  traces  of  it  are  to  be  found  throughout  Hfe  in  the 
centre  of  the  intervertebral  discs.  When  fully  developed  it  is  a  cylindrical  rod 
composed  of  clear  epithelium-like  cells,  enclosed  within  a  special  sheath  of  homo- 
geneous substance.  These  cells,  although  they  may  become  considerably  enlarged 
and  vacuolated,  undergo  no  marked  histogenetic  change  and  tjike  no  part  in_  the 
formation  of  any  tissue  or  organ  of  the  adult. 

~  Separation  of  the  enilt)ry;o  from  the  blastoderiii. — The  embryonic  rudiment 
which   thus   first   makes   its   appearance   in   the   blastoderm,  and   which   consists 


coeLoiro 


Fis. 


35. — Mesial   sagittal   sectioi;    through    the    anterior   end    of 
showing  the  commencing  formation  of  the  fore-gut. 


i.N     EARLY     SHEEP 

(Bonuet. ) 


cp,  epiblast ;  liy,  hypoblast  ;  /..'/.,  foregut,  formed  by  folding  over  of  layers  ;  am,  amnion  (head  fold). 
Below  the  foregut  the  cephalic  ccelom  is  becoming  formed  as  clefts  in  the  niesoblast. 

essentially  of  neural,  ^groove,  mesoblastic  thij3keniuos,  and  notochord,  becomes 
at  a  very  early  period  marked  off  in  front  by  a  dipping  down  omTe1)lastodermic 
layers  immediately  in  front  of  the  anterior  end  of  the  neural  groove,  so  as  to  form  a 
transverse  curvilinear  sulcus — the  ankvioj\UmiliRg.  Srulcvs.  This  is  at  first  wide 
and  shallow,  but  soon  deepens  and  narrows,  and  takes  at  the  same  time  an  oblique 
direcfioh,  curving  downwards  and  backwards  under  the  front  end  of  the  neural 
groove.  The  sulcus  is  really  due  to  a  growth  forwards  of  the  anterior  end  of  the 
eiiibi'yonic  rudiment,  over  the  part  of  the  blastoderm  immediately  in  front  of  it,  so 
that  this  anterior  end  now  projects  as  a  distinct  head  (fig.  35). 

All  the  three  layers  are  involved  in  the  forward  growth  and  overfolding  which 
produces  the  head,  so  that  a  prolongation  from  the  blastodermic  cavity,  which  is  of 
course  lined  by  hypoblast,  becomes  included  in  the  head,  and  the  anterior  part  of 
the  primitive  alimentary  canal,  oi fore-gut  (f.g.)  is  thereby  produced.  Formed  in  this 
way  its  front  end  is  necessarily  blind,  and  for  a  long  while  there  is  no  mouth  nor  any 
communication  between  the  fore-gut  and  the  exterior  of  the  embryo.  The  niQuth. 
becomes  formed  later  by  invagiimtion  from  the  exterior. 

In  the  rabbit,  and  also  in  the  chick,  the  blastoderm  at  this  time  is  still  bilaminar  in  and 
near  the  middle  line  in  front  of  the  embryo,,  for  the  growth  of  the  mesoblast  has  not  yet 


FORMATIOX    OF    I'lUJ-AMNIONT. 


;35 


exteniled  to  this  part.  The  head  tml  of  the  einbryo  arrows  forward  over  this  hibiininar  portion, 
and  since  the  embryo,  as  it  becomes  <lill"erentiated,  tends  to  sink  below  the  treneral  surface  of 
the  bhistoderm.  the  head  which  now  overlies  the  bibiniinar  purt  proiluces  a  depression  of  this 
part  towards  the  interior  of  tho  vesicle,  so  that  the  head  of  the  embryo  becomes  enclosed  by 
the  bilamiiiar  wall  uf  the  depression  (fi;r.  IJfi)-  The  enclosing,'  membrane,  which  is  well  marked 
in  the  rabbit,  has  been  termeil  by  v.  Heneden  the  pro-am  n'lon  :  l>y  an  extension  of  mesoblast  and 
of  the  mesoblastic  cleavaiie  between  its  layers,  it  afterwards  becomes  split  into  .-^omatoiileure 
and  splanchnoi)leure.  and  the  former  l)ecomes  continnous  with  tlie  true  amnion  (see  p.  42). 
The  stage  of  pru-amniou,  if  it  exists  at  all,  must  disappear  very  early  in  the  human  embryo. 

Soon  after  the  appoanince  of  the  anterior  limiting  sulcus,  byp  lateral  limiting 
sulci  are  seen  ruiinin<;-  external  and  parallel  to  the  medullary  fulds  ;  these  lateral 
sulci,  as   they  dip  down,  mark  ott"  the  body  of  the  embryo  from  the  rest  of  the 


Fig.    36.— Diagrammatic    loxgitudixal    sections    through    the    embryo   of   the    rabbit.      The 

SECTIONS  SHOW  THE  MAXXER  IX  WHKH  THE  PRO-AMXIOX  IS  FORMED  BY  A  DIPPIXG  DOWN  OF 
THE  HEAT)  AND  AXTERIOR  PART  OF  THE  BODr  IXTO  A  DEPRESSION  OF  THE  BLASTODERM,  WHICH 
AT  THIS  PART  IS  FORMED  OF  KPIBLAST  AXD  HYPOBLAST  ONLY.  ThE  DIAGRAMS  ALSO  ILLCSTRATE 
THE     MODE    OF    FORMATION     OF     THE     ALLANTOIS     AXD     OF     THE     TAILFOLD     OF     THE     AMNION    IN    THIS 

ANIMAL,     (v.  Beneden  and  Julin. ) 

ejt,  epiblast  ;  luj,  hypoblast  ;  me,  mesoblast ;  cce,  parts  of  the  cwlom  ;  coe',  pericardial  ccelom,  the 
heart  not  being  represented  ;  'pr.a.,  pro-amnion  ;  pi,  seat  of  formation  of  the  placenta  ;  all,  allantois  ; 
am,  amnion. 

blastoderm,  but  they  do  not  for  some  time  progress  fiir  in  development,  the  middle  part 
of  the  future  alimentary  tract  long  remaining  in  free  continuity  with  the  cavity  of  the 
blastodermic  vesicle  (fig.  45,  and  fig.  40,  d),  but  becoming  gradually  more  pinched  off 
from  it.  That  part  of  the  cavity  of  the  original  blastodermic  vesicle  which  does  not 
form  a  part  of  the  alimentary  canal,  but  remains  connected  with  it  by  a  wide  neck 
of  communication,  is  known  as  the  ijolk-sac.  At  a  later  stage,  when  the  body  walls 
are  formed,  and  the  yolk-sac,  relatively  greatly  diminished  in  size,  lies  altogether 
outside  the  body  of  the  foetus,  it  is  merely  connected  by  a  long  narrow  duct,  which  runs 

D  2 


36 


CLEAVAGE    OF    THE    MESOBLAST. 


along  the  umbilical  cord,  with  the  intestine.    This  is  termed  the  umhilkcd  duct,  and  the 
yolk-sac  itself  has  received  (in  mammals)  the  name  oiumhilical  vesicle  (figs.  50,  51). 
Lastly,  at  the  tail  end  of  the  embryo  a  hinchgid  is  produced.     In  the  human 
embryo  this  appears  to  be  formed  by  a  protrusion  from  the  posterior  blind  end  of  the 
enteric  groove,  and  after  the  formation  of  the  allantoic  tube,  but  in  most  mammals 


Fig.  37. — Transverse  section  of   the   tail   end   of.  an   embrto   chick   of   the  latter  half  of 

THE   second    DAT,    AT    THE    PLACE    WHERE    THE    VERTEBRAL    SOMITES    CEASE.       \^       (From  Kolliker. ) 

m.f.,  medullary  folds,  the  neural  canal  beginning  to  close  ;  jy-m.,  paraxial  mesoblast  ;  l.in.,  lateral 
mesoblast ;  ep. ,  epiblast  ;  hy,  hypoblast ;  ao,  primitive  aorta ;  cJi,  notochord  ;  p,  ccelomic  cleavage  of 
lateral  plate  of  mesoblast. 

and  in  birds  it  is  produced  by  a  folding  over  of  the  tail  end  of  the  embryo  like  that 
which  occurs  at  the  head  end  to  enclose  the  fore-gut  (figs.  45  to  47).     Tliiejiincl-gut 


Fig.  38. — Embryo  chick  at  the  end  of  the  second  day,  seen  from 
BELOW,     f.     (From  Kolliker.) 

V7i,  forebrain  ;  A!),  primary  ocular  vesicles  ;  CJi,  notocliord  ;  //,  tubular 
heart ;  om,  vitelline  veins  ;  Vd,  entrance  to  the  forepart  of  the  alimentary 
canal  within  the  cephalic  fold  ;  in  the  middle  part  of  the  embi-yo,  the 
protovertebral  somites  are  seen  (to  the  number  of  thirteen  pairs)  on  each  side 
of  the  canal  of  the  spinal  maiTOw  and  notochord. 

remains  for  a  considerabje  time  blind,  until  the  anus  becomes 
formed  by  invagmation  from  the  ezterior. 

Cleavage  of  mesoblast.     Formation  of  body  cavity. — 

At  a  very  early  period,  soon  indeed  after  the  formation  of  the 
neural  groove,  two  important  changes  begin  in  the  mesoblast. 
One  of  these  is  the  cleavage  of  the  lateral  mesoblast  (which  is  at 
first  a  continuous  sheet)  into  two  plates,  one  of  which  clings  to 
the  epiblast,  and  the  other  to  the  hypoblast.     The  cleft  is  at  first 
small  (fig.  37,  p),  but  accumulation  of  fluid  within  it  soon  con- 
verts it  into  a  cavity,  which  gradually  spreads  until  the  separation 
is  very  extensive  (fig.  39,  j^.^J.)-     The  layer  of  mesoblast  which 
clings  to  the  epiblast  eventually  forms  part  of  the  body-wall, 
and  is  known  as  the  somafopleiiral  mesoblast;  that  which  clings 
to   the   hypoblast   forms   eventually  part  of  the  wall  of   the 
alimentary  tract,  and  is  known   as  the   sj^lanch/wpleural  mesohlast.     The  cavity 
between  these,  which  is  formed  by  enlargement  of  the  original  cleft,  is  the  ccclom  or 
lody  cavity  fplenro-peritoneal  canity  of  authors). 

Formation  of  mesoblastic  somites. — The  other  change  occurs  not  in  the 
lateral  but  in  the  paraxial  mesoblast,  and  consists  in  the  occurrence  at  regular 
Intervals  transversely  along  the  mass,  of  a  process  of  thinning  which  produces  its 
complete  separation  into  distinct  segments,  so  that  when  the  embryo  is  viewed  from 
above  or  below,  these  segments  appear  on  either  side  of  the  neural  groove  as  a 
linear  series  of  small  quadrangular  masses  (fig.  38),  which  were  originally  termed 


FORMATION    OF    MESOBLASTIC    SOMITES. 


37 


proiovnishrfSi  on  tlic  supposition  (now  known  to  hu  yroncous)  that  they  iire 
the  rudiments  of  the  future  vcrtebroc  ;  they  arc  more  appropriately  termed  the 
mesohlastis^mjlrs.  "~ 


Fig.  30. — Transverse  section  through  the  dorsal  region   of   an   embryo  chick  op  45  hours. 

(From  Balfour.) 

A,  epiblast ;  C,  hypoblast  ;  Mc,  medullary  canal  ;  Pv,  protovertebra  or  mesoblastic  somite  ;  Wd, 
intermediate  cell-mass  in  which  the  Wolffian  duct  is  becoming  formed  ;  <S'o,  somatoplenre  ;  Sp, 
splanchnopleure  ;  pp,  pleuro-peritoneal  cavity  (ca;lom)  ;  op,  inner  margin  of  arcxi  opaca  ;  w.  thickened 
hypoblast  of  area  opaca  ;  ao,  left  primitive  aorta  ;  v,  blood  vessels  ;  ch,  notochord. 


In  AmphioxHs  (figs.  28,  20),  the  protovertebra3  are  formed  in  common  with  the  body  cavity,  and 
are  successively  separated  off  from  before  baclrwards  from  the  coelomic  fold  as  hollow  cuboid 
somites,  each  of  which  extends  upwards  around  the  neural  canal  and  downwards  along  the 
sides  of  the  alimentary  canal,  and  sub.sequently  divides  into  a  dorsal  or  paraxial  part  which 
forms  the  protovertebrte,  and  a  ventral  part  which  forms  the  lateral  mesoblast.  At  first  the 
hollow  somites  communicate  individually  with  the  alimentary  cavity,  but  they  Ijecome  shut 
off  from  this  long  before  the  division  which  has  just  been  mentioned.  The  ventral  segments 
run  together  eventually,  to  f  onn  a  continuous  serous  cavitj'.  In  sections  of  bird  or  mammalian 
embryos  (fig.  30),  the  protovertebra^.  although  on  the  whole  compact  masses  of  meso- 
blast, yet  often  show  a  tendency  to  have  their  central  cells  loosely  an-anged,  so  as  to  give  the 
appearance  of  an  irregular  cleft  in  their  interior,  and  sometimes  a  definite  cavity  is  formed  in 
them,  which  may  even  be  continuous  with  the  coelomic  cleft  in  the  lateral  mesoblast. 

Protovertebras  begin  to  be  marked  off  in  the  paraxial  mesoblast,  a  short  distance 
from  the  anterior  end  of  the  neural  groove,  in  what  will  eventually  become  the 
cervical  region  of  the  embryo.  They  are  produced  in  succession  from  before  back- 
wards, one  or  two  only  being  at  first  visible  on  either  side,  and  others  being  gradually 
added  as  the  embryo  gi'ows  in  length,  until  a  large  number  may  at  length  be  counted, 
extending  from  immediately  behind  the  cephalic  region  to  the  region  of  the  primitive 
streak. 

Cerebral  vesicles. — Meanwhile  a  change  of  importance  has  taken  place  in 
connection  with  the  antei'ior  .end  of  the  neural  groove,  which  has  become  enlarged, 
and  S'  I  >\\  exhibits  a  succession  of  highly  charactefistic  median  dilatations,  separated 
from  ont-  another  by  slight  constrictions  (fig.  40).  These  dilatations,  at  a  later 
stage.  ulUr  thoy  Iiuac  become  roofed  in,  along  with  the  rest  of  the  neural  groove, 
are  known  as  the  cerebral  resides.  There  is  at  first  a  simple  enlargement,  and 
behind  this  two  others  form  in  succeasion,  so  that  the  p/;««;v/  resides  are  tlu-ee  in 


38 


CEREBRAL    VESICLES. 


number,  but  subsequently  the  most  antexlor  {fore-brain)  and  posterior  (Jnnd-hrain) 
e^chjforms  two  yesicles,  whereas  the  middle  vesicle  (mid-irain)  remains  permanently 
undivided._  Five  vesicles  are  therefore  theiTtoTe'seen,  and  these  giv_e  rise  eventually 


Fis 


40. —Surface  view  of  an  early  embryo  op  the  guinea  pig  showing  the  commencement  op 
the  three  primary  cerebral  a'esicles  (1,  2,  3)  as  enlargements  of  the  medullary  groote. 
Semi-diagrammatic. 

2}r,  primitive  streak  and  groove. 


Fig.  41.— Rabbit  embryo  of  the  9th  day,  from  the  surface.     -J.     (Kolliker.) 
Tlie  medullary  groove  is  enlarged  anteriorly  and  the  primary  optic  vesicles  are  growing  out  from 


the  first  cerebral  enlargement.     On  either  side  of  the  head,  the  (double)  tubular  heart  is  seen. 
l^airs  of  j)rotovertebrse  are  formed. 


Eight 


to  the  five  fmidamental  divisions  of  the  brain,  while  from  the  sid^s  of  the  fore-brain 
the  rudiments  of  the  optic"  nerves  and  retinse  grow  out  as  hollow  protrusions. 

HeaiSr  and  vascular  system. — While  this  change  is  progressing  in  the 
neural  canal,  and  the  proto\ertebr9e  are  becoming  formed  in  the  paraxial  mesoblast, 
the  first  sign  of  a  vascular  system  is  beginning  to  make  its  appearance  in  the  meso- 
blast on  either  side  of  the  head  in  the  form  of  a  simple  tubular  vessel  (fig.  41),  Avhich 
becomes  developed  in  the  splanchnopleure  in  this  srEfeiiroiT."  Xs.  the  splanchnopleural 
mesoblast  and  its  accompauying  hypoblast  fold  round  on  either  side  under  the 
head  to  form  the  fore-gut,  these  two  simple  tubes  necessarily  come  together  in  the 
middle  line,  and  they  then  fuse  together  Ion gitjjdin ally  to  form  a  sjngle^tube,  the 
priinitive  heart  (fig.  38) ;  this  tube  runs  for  a  short  distance  in  the  mesoblast  immediately 
under  tlie  fore-gub,  and  then  divides  into  two  branches,  Avhich  pass  laterally,  so  as  to 
partially  encircle  the  fore-gut,  and  thence  course  backAvards  along  the  body  of  the 
embryo  on  either  side  of  the  notochord.  These  two  vessels  form  the  primitive 
arteries  (primU'ive  aortcc),  the  part  of  each  which  encircles  the  fore-gut  as  it  passes 
dorsalwards  being  known  as  Vaefi'nt  aortic  arch.     On  the  other  hand,  the  posterior 


HKAUr    AM)    VASCULAR    SVSTKM. 


39 


cud  of  tlie  single  tubular  lifiirt  bifurcates  at  an  obtuse  anji^lc  to  form  two  lar<je 
venous  roots  {j>n'tti_ilire  I'ci/i.s),  wlit-h  receive  the  blood  from  tb.e  vascular  area  on  tlie 
yolk-sac  when  this  is  developed,  and  juiss  it  on  to  the  heart,  'i'hese  two  primitive 
veins  become  the  ritcUine  reins. 

The  heart  begins  to  beat  very  soon  after  its  ajipearanc.'!',  e\L'n  whilst  still  (illed 
only  with  a  colourless  fiuid,  and   before  receiviuL;-   blood   from   the  va-scuiar  area. 


Fig.  4-2. — Vascular  area  of  ihe  rabbit  of  10  days.     (v.  Beneden  and  Julin.) 
The  arteries  and  arterial  capillaries  are  represented  red.  the  venous  capillaries  and  veins  blue. 


Afterwards,  when  receiving  and  propelling  the  red  blood  from  that  area,  and 
especially  after  it  has  become  elongated  and  bent  upon  itself,  it  is  one  of  the  most 
prominent  objects  seen  on  examining  the  embryo  ;  projecting  as  it  does  freely  into 
the  yet  widely  open  coeloni  immediately  behind  and  beneath  the  cephalic  region  of 
the  body. 

The  first  vessels  to  be  developed  are  formed  in  mesoblast  altogether  outside  the 
body  and  ^yithin  a  circular  area  (vascular  area),  which  sun-ounds  the  devek»ping 
embryo  for  a  certain  distance.  The  first_appearance  of  red  blood  occurs  in  the  form 
of  isolated  red  points  (?>/oor/-/67f///^/6-  of  Pander),  which  are  scattered  about  within 
this  area,  and  are  especially  numerous  at  its  circumference,  where  they  form  an 


40 


HEART  AND  VASCULAR  SYSTEM. 


almost  continuous  chain.  These  red  points  are  small  groups  of  colom'ed  nucleated 
blood  corpuscles  which  have  been  developed  within  certain  of  the  mesoblast  cells  in 
the  manner  explained  in  another  portion  of  this  work  (see  Histology,  Development  of 
blood-corpuscles  and  blood-vessels).  The  mesoblast  cells  in  question  form  the  blood- 
vessels of  the  vascular  area  by  becoming  united  with  one  another  into  a  capillary 
network,  which  becomes  connected  mesially  with  branches  of  the  primitive  aortfe 


Fig.   43. — Vascular  area  op  the  rabbit  of  11  days.     (v.  Beneden  and  Julin. ) 

The  arteries  are  represented  red,  the  reins  Lhie  ;  the  capillaries  are  not  shown. 
In  both  the  stages  illustrated,  the  terminal  sinus  is  seen  to  he  arterial. 


(vitelline  arteries),  and  peripherally  with  a  circular  vessel  {termincd  sinus),  arterial  in 
mammals  but  venous  in  the  chick,  which  forms  the  circumferential  boundaiy  of  the 
vascular  area.  From  the  capillary  network  of  the  vascular  area  the  blood  is  collected 
into  two  vitelline  veins,  which  course  backwards  and  inwards  to  carry  the  blood  of  the 
area  to  the  venous  roots  of  the_ heart.  This  is  the  first  circulation,  or  the  circulation 
of  the  vasculaflirea.  It  is  also  called  the  Vitelline  circulation,  because  the  vascular 
area  is  developed  in  the  mesoblast  of  the  splanchnopleural  layer  which  encloses  the 
vitellus,  and  its  capillaries  are  an  important  means  of  bringing  the  food  material  of 
the  vitellus  to  the  embryo.  ■^■— 

The  origin  of  the  cells  which  form  the  endothelium  of  the  vascular  system  is  not 
definitely  known.  It  has  been  supposed  by  some  that  they  are  in  the  first,  instance  of 
hypoblastic  origin,  but  the  evidence  on  this  point  is  insufficient,  and  it  is  more  generally 
held  that  they  are  specially  modified  mesoblastic  cells.  Regarding  the  origin  of  the 
endothelium  of  the  heart,  see  p.  136. 


RECENT    LITERATURE.  41 


RECENT    LITERATURE.* 

Beneden,  v.,  and  Julin,  Recherchea  sur  Ja  formation  dea  annexes  falales  rluz  let  mammifirea 
{Ijopin  et  L'heiropteres).  Arch,  tie  biologic,  t.  v.,  1884. 

Carius,  Ueber  die  Enturicklung  der  Chorda  u.  dtr  primitiven  Raclttnkaut  bet  Meertdiweinchen 
und  Kauinclien.  Inaug.  Diss.,  Warburg,  1888  :  Vein-  den  Kopfforlsatz  dea  Kaninehena.  Marburg 
Sitzungsbcr,  1887  ;   Ueler  die  AualAlduiuj  dcs  hinttren  Korperendes  lei  Cavia.     Ibid.,  1888. 

Chiarug^i,  Anatomic  d'un  embryon  humain  de  la  lowjueur  de  mm.  2  6  en  ligne  droUe.  Arch, 
ital.  de  biologic  XII. 

Ehlers,  E.,  Nebendarm  und  Chorda  dorsalia.  Nachrichten  d.  kgl.  Gesellscb.  d.  Wissensch.  za 
Gottingen,  1885. 

Fol,  H.,  Deacription  d'un  embryon  humuin  de  cinq  millimetrea  et  six  dixiimu.  Recueil 
Zool.  Suisse,  1884  ;  Rccherches  sur  le  ddveloppcment  dta  protoverUbrea  chez  V embryon  du  pouUt. 
Archives  des  sciences  physiques  et  naturelles,  1884. 

Giacomini,  Sul  canale  neiLrentcrico  et  sul  canale  anale,  <Lc.  Giom.  de  r.  accad.  di  medic,  di 
Torino,  1SS8. 

Keibel,  F.,  Zur  EntwicUungsgesch.  der  Chorda  bei  Sdugem.  Arch.  f.  Anat.  u.  Physiol.,  Anat. 
Abth.,  18S9. 

Eolliker,  v.,  Ueber  die  Chordahohle  und  die  Bildung  der  Chorda  beim  Kaninchen.  Sitznngsber. 
der  physiciil.-med.  Ges.  in  Wiirzburg,  1883. 

liieberkulm,  N.,  Ueber  die  Chorda  bei  Sdugethieren.  Arch,  fiir  Anat.  und  Physiol.,  Anat. 
Abth.,  1884. 

Perenyi,  J.  v.,  Entuncklung  des  Am,nion,  des  Wolff^schen  Ganges  und  der  AllanUns  bei  den  Repti- 
lien.    Zool.  Anzeiger,  No.  274,  1888. 

Eavn,  Ueber  die  mesoderm/reie  SteUe  in  der  Keimschcibe  des  Hiihnerembryo.  Arch.  f.  Anat., 
ISS6. 

Homiti,  G.,  Be  Vextremite  antirieur  de  la  corde  dor-:ale  et  de  son  rapport  avec  la  poche  hypophy- 
taire  ou  de  Eathke  chez  Pembryon  du  poulet.     Archives  italiennes  de  biol.,  t.  VII.,  1886. 

Shore  and  Pickering',  The  pro-am,nion  and  amnion  in  the  chick:  Journal  of  Anatomy  and 
Physiology,  1SS9. 

Solger,  B.,  Studien  zur  Entwicklungsgc^chichte  des  Ccdom^  und  des  Ccelomepithels  der  Amphibien. 
Morphol.  Jahrbuch  X.,  18S5. 

Spee,  F.  v.,  Ueber  die  Entwicklungsvorgdnge  vom  Knoten  aus  in  Sdugethicrkeim^cheiben.  Anat. 
Anzeiger  iii.,  1888  ;  Beobachtungen  an  einer  menschlichen  Keimscheibe  mit  offener  Medullarinne  u. 
Canalis  neureutericus.     Arch.  f.  Anat.  u.  Physiol.,  Anat.  Abth.,  1889. 

Strahl,  H.,  Ueber  die  Enticickelung  des  Canalis  myelo-entericus  und  der  Allantois  der  EidecJise. 
Archiv  fiir  Anat.  und  Physiol.,  Anat.  AbtheU.,  1881  and  1883. 

"Works  dealing  with  the  general  subject  of  Embryonic  Formation  and  Development. 

Balfour,  A  Treatise  on  Comparative  Embryology.    London,  1880. 

Foster  and  Balfotir,  The  Elements  of  Embryology,  Second  Edition,  1883.  Edited  by 
Sedgwick  &  Heape. 

Haddon.     An  introduction  to  the  study  of  Embryology.     London,  1887. 

Hert-wig,  O.,  Lehrbuch  der  Entvncklungsgeschichte  des  Menschen  u.  d.  Wirbelthiere.  2te 
Auflage,  Jena,  1888. 

His,  "W.,  Uniersuckungen  ue.  d.  erste  Anlage  d.  Wirbelthierleibes.  Die  erste  Enticicklung  dea 
Eiihnchens  im  Ei.  Leipzig,  1868  ;  Unsere  Korpeiform  u.  das  physiol.  Problem  ihrer  Enstehung, 
1871  ;  Anatomie  menschlicher  embryonen,  1881 — 1885. 

Kolliker,  v.,  Eniwicklungsgeschichte  des  Menschen  u.  d.  hoheren  Thicre.  2te  Auflage.  Leipzig, 
1879. 

Eauber,  PormhUdung  «.  Formstorung  in  der  Enticicklung  von  Wirbelthieren.  Morph.  Jalirb. 
vi.,  1880. 

*  The  early  changes  in  the  blastodenri*;  layers  are  dealt  with  in  several  of  the  papers  enuracrateo 
on  pp.  27  to  29. 


4S 


DEVELOPMENT    OF    THE    FCETAL    MEMBRANES. 


DEVELOPMENT     OF    THE    PCETAL    MEMBRANES;    ATTACHMENT 
OF     OVUM    TO    UTERUS. 

Having  thus  sketched  out  the  manner  in  which  the  principal  organs  of  the 
body  first  make  their  appearance,  we  may  briefly  consider  the  formation  of  certain 
structures  which  have  a  purely  embryonic  existence,  and  are  concerned  either  with 
the  nutrition  of  the  foetus  and  its  attachment  to  the  mucous  membrane  of  the 
uterus  (chorion,  allantois,  placenta),  or  serve  the  purpose  of  protecting  the  embryo 
against  mechanical  injuries  by  suspension  in  a  bag  of  fluid  (amnion). 

Formation  of  the  amnion  and  chorion. — The  amnion,  which  is  only  found 
in  reptiles,  birds,  and  mammals  (amniota),  is  a  membranous  bag  occupied  by  a  clear 
albuminous  fluid,  and  covers  the  whole  of  the  embryo.    It  is  developed  from  folds  of 


false  ocmjvtcn'  or  chorlOTi^ 


•viliL  of 


Fig.  44. — Diagram   of    a    transverse    section   of    a   mammalian    embryo    showing  the  mode  of 

FORMATION    OF    THE    AMNION.       ThE    AMNIOTIC    FOLDS    HAVE    NEARLY    UNITED    IN    THE    MIDDLE    LINE. 

Epiblast,  blue  ;  mesoblast,  red  ;  hypoblast  and  notochord,  black. 

somatopleure,  which  are  reflected  from  the  head  and  t^iiLends  and  lateraL  boundaries 
ofThe  embryo  at  an  early  stage.^  With  the  sinking  of  the  embryo  into  the  blasto- 
dermic vesicle  or  yolk,  these  folds  grow  up  over  the  back  (fig.  44)  until  they  meet  and 
coalesce  with  one  another  along  the  middle  line,  in  such  a  manner  as  to  form  two 
distinct  membranes,  one  of  which,  the  inner,  is  the  amnion  (true  amnion),  while  the 
outer  membrane  (termed  the  false,  amnion)  becomes  applied  to  the  greatly  thinned 
remnant  of  the  zona  pellucida,  and  eventually  forms  a  complete  external  covering 
to  the  ovum  and  its  contents.  This  external^covering  of  the  ovum  has  been  long- 
known  as  the  chorion — a  name  which  iias7however,  been  applied  by  some  embry- 
ologists   to    othSF-itructures.2      It   is   fixed   to   the   uterus   by  vilh,   which   are 

^  The  head  fold  is  preceded  at  a  yet  earlier  stage  by  the  bilamiiiar  pi'O-amnion,  the  formation  of 
which  has  been  already  alluded  to  {]).  35). 

"  The  term  "  chorion"  has  been  applied  to  various  structures  by  embryologists.  Originally  used  to 
denote  the  external  covering  of  the  developing  ovum,  it  was  emi^loyed  successively  for  the  zona  pellucida 
("  primitive  chorion  "),  the  epithelial  enclosing  membrane  of  the  blastodermic  vesicle,  and  finally  for 
the  external  amniotic  fold  or  false  amnion,  when  this  becomes  formed.  Lately  it  has  been  used  to 
express  the  external  albuminous  enveloije  of  the  undeveloped  ovum,  so  that  it  is  probable  that 
much  confusion  may  arise  unless  the   meaning   of   the   term  be  in   each  case  clearly   defined.      It 


l>KVKLur.\lKXT    OF    THE    FaMAL    .MKMHUANKS. 


43 


iittaclit'd  to  the  utoriiit'  inucous  lueinbraue,  aiul  these  vijlj^siiljscmiciitly  become 
nijiiilied  and  vascular  when  the  {rrowth  of  the  aUantuis  has  broujfht  the  uiubJJical 
blood-vessels  to  the  chorion  ;  but  cxc^UiLJH  the  Dhtccnta,  they  at  len^'th  all  become 
atroi>riTecI  and  disuppcar. 

It  will  i)e  seen  from  the  manner  in  which  the  true  and  false  amnion  arc  formed 
by  a  fold  of  somato|)leure,  that  these  membranes  are  composed  of  both  epiblast  and 
mesoblast.  In  the  falsejiuinion  the  epUilast  becomes  the  oiiterja^er ;  in  the  true 
i\""iion  it  is  the  innei-jb^i;.  The  mes(.blast  of  the  one  is  sei)anited  from  that  of 
the  other  by  a  space  occupied  by  fluid,'  and  continuous  with  the  ccelom,  with  which 


■fcn'&-atvt 


Fig.  45. — Diagram  of  a  longitudinal  section  of  a  mammalian  embryo,  after  the  completion 

OF    THE    AMNION. 

in  fact  it  remains  continuous  until  the  body  walls  of  the  embijo  have  entirely  gi'own 
round  and  coalesced  on  the  ventral  surface — the_  final  point  of  coalescence  being  the 
umbilicus.  With  this  enclosing  growth  of  the  l^ody  walls  the  line  of  reflection  of 
the  ainniotic  fold  is  also  carried  downwards,  so  that  the  amnion  is  eventually 
attached  around  the  umbilical.. cord,  by  which  the  fcctns  appeai-s  suspended  in  the 
anmiotic^iiTdXEg.  oO). 

In  the  guinea-pig.  in  wMcli  the  epiblast  is  the  innermost  layer,  the  amnion  is  not  formed 
as  a  fold,  but  results  from  an  extension  of  the  mesoblastic  cleavage  around  the  dorsal  a.spect 
of  the  central  cavity  ;  this  cavity  thus  becomes  the  cavity  of  the  amnion. 

Fqrm.€.ticuio£  the  allantois. — Both  the  amnion  and  the  chorion  are  entirely 
extra-embryonic  structures,  i.e.,  they  are  external  to  the  body  of  the  embryo,  and, 

\vill  be  used  throughout  this  article  in  the  sense  in  which  it  has  hitherto  almost  invariably  been 
employed  in  human  embryology,  to  denote  that  external  membrane  of  the  ovum  from  which  the 
villi  (chorionic  villi)  which  grow  into  the  uterine  mucous  membrane  spring,  and  this  it  will  be  seen 
presently,  is  that  part  of  the  external  investment  of  the  blastodermic  vesicle,  v.hich,  when  the  amnion  is 
formed,  becomes  the  external  amniotic  fold  or  false  amnion. 

^  Except  in  the  later  stages  of  gestation,  when  the  amniotic  and  chorionic  mesoblast  become  loosely 
united  by  jelly-like  connective  tissue. 


44  FORMATION    OF    THE    ALLANTOIS. 

although  they  minister  to  its  protection  and  nutrition,  take  nqjwfc  in  the  formation 
of  any  of  its  organs.  But  tEe~case  is  different  with  the  structure  next  to  be 
described,  viz.,  the  allantois,  a  part  of  whicii  does  in  fact  eventually  become 
converted  into  portions  of  ithe  urinary  and  generative  systems,  although  the  greater 
part  is  also  extra-embryonic,  its  function  being  to  minister,  through  its  accompanying 
blood-vessels,  to  the  nutritive  and^'respiratory  exchanges  of  the  foetus. 

Tlie  time  of  development  of  the  allantois  seems  to  vary  much  in  mammals,  and 
there  is  reason  to  believe  that  it  is  found  in  the  human  embryo  at  a  very  early 
period— indeed  the  earliest  human  embryos  that  have  hitherto  been  described 
already  possess  an  allantois.  In  most  animals  in  which  its  development  has  been 
studied  the  allantois  has  been  found  to  begin  as  a  hollow  prolongation  of  the  posterior 
end  of  the  primitive  alimentary  canal  (fig.  46).     It  soon,  however,  becomes  relatively 


Fig.   46. — Longitudinal  section  through  the  posterior  end  of  an  embryo  rabbit,  showing  the 

OUTGROWTH  OP  THE  ALLANTOIS.   (KolHker. ) 

h,  epiblast  of  trunk  ;  del,  hypoblast  ;  m,  medullary  or  neural  canal  ;  ch,  notocliord  ;  hd, 
commencing  hind-gut,  which  is  becoming  formed  by  a  folding-over  of  the  tail  end  of  the  embryonic 
blastoderm,  v  ;  ed,  blind  end  of  hind  gut  ;  al,  allantois  gi'owing  out  from  hind  gut  into  aw,  mesoblastic 
thickening  ;  e,  epithelium  of  yolk  sac  ;  df,  splanchnopleure  ;  hp,  somato-pleure  passing  superiorly  into 
am,  tail  fold  of  amnion  ;  s,  is  placed  within  the  cavity  of  the  amnion,  and  denotes  the  tail  end  of  the 
embryo. 

shifted  in  position  so  as  to  come  off  from  the  ventral  wall  of  the  hind-gut,  growing 
into  the  posterior  extension  of  the  mesoblastic  cleft  and  eventually  into  the  space 
between  the  false  and  true  amnion,  and  carrying  along  with  it  its  mesoblastic  covering 
(figs.  36,  45),  It  is  therefore  composed  eventually  of  two  parts,  viz.,  (jj  a  hypo- 
blastic  sac  which  communicates,  at  fu'st  widely  but  afterwards  by  a  narrowed  orifice, 
with  the  hind-gut,  and  (2)  an  investment  of  mesoblast.  This  last  is  usually  greatly 
thickened  and  very  vascTilar,  and  is  directly  supplied  with  blood  by  two  arteries 
(allantoic  or  umbilical  arteries),  which  appear  at  first  as  a  direct  continuation  of 
the  primitive  aortee.  As  the  allantoic  vesicle  expands  into  the  cavity  of  the  false 
amnion,  it  carries  the  vascular  mesoblast  along  with  it,  so  that  this  mesoblast  is  thus 
brought  to  the  inner  surface  of  the  chorion,  over  which  its  blood-vessels  then  spread 
so  as  to  convert  this  hitherto  non-vascular  membrane  into  one  which  is  richly 
supplied  with  blood-vessels.  The  chorion  has  grown  in  the  form  of  ramified 
villi  into  the  substance  of  the  uterine  mucous  membrane  or  decidua  even 
before  this  advent  of  the  vascular  tissue  of  the  allantois,  but  the  chorionic  villi 
now  receive  blood-vessels  and  thus  become  vascularized,  the  interchanges  between 
the  foetal  and  maternal  vascular  systems,  which  are  afterwards  confined  to  one  region 
only  of  the  chorion  and  decidua — that  which  forms  the  placenta^occurring  in  the 
first  instance  over  the  whole  superficies  of  the  ovum. 


FORMATION    OF   THE    ALLANTOIC. 


-1-5 


Like  the  amnion,  an  allantois  is  only  formed  in  the  embryo  of  reptilcH,  birtlH,  and  mammals.' 
It  varies  chiefly  in  tho  extent  to  wliicli  its  liypoblastic  part  liceomcH  developed.     In  reptiles, 


Fig.  47. — Longitudinal  section  at  a  slightly  later  stage  than  that  suuwn  in  kiu.  4ti. 

The  allantoic  protrusion  now  springs  frnin  the  ventral  wall  of  the  hind  gut.     Lettering  as  in  fig.  4(5. 
(FroniKolliker.) 

Fig.   48. — Early  human  embryo.     (From  His  after  Coste.) 

The  embryo  is  enclosed  within  the  amnion  am,  and  is  attached  at  its  caudal  end  by  the  allantoic 
stalk,  all,  to  the  chorion.     The  yolk  sac,  ys,  is  still  distinct  from  the  allantoic  stalk. 


Fig.  49. — Dl.VGUAM  OF  longitudinal  sections  THUO0GH  THE  HUMAN  (JVUM  AT  SUCCESSIVE  STAGES 
SHOWING  THE  DIPPING  DOWN  Of  THE  EMBUVO-RUDIMEXT  INTO  THE  BL.VSTODERMIC  VESICLE,  AND 
THE  FORMATION  OF  THE  FOKEGUT,  AMNION,  AND  STALK  OF  THE  ALLANTOIS  BY  THE  FOLDING  OF 
THE    BLASTODER.M.       (His.) 

Am,  head  fold  of  the  amnion  (pro-amnion  in  b  and  c)  ;  .Y^,  yolk  .sac.  a  and  d  are  conditions  of  the 
embryo  which  have  been  seen  and  described  ;  b  and  c  are  intended  to  show  how  the  conditions  found 
in  (I  may  be  brought  about,  and  especially  how  the  stalk  of  the  allantt)is  nuiy  be  regarded  as  a  direct 
continuation  of  the  posterior  end  of  the  embryo,  which  accoi'ding  to  His  does  not  lose  at  any  time  its 
connection  with  the  chorion  or  villous  external  membrane  of  the  ovum.  The  curved  dotted  lines  in  c 
indicate  the  formation  of  the  amnion  and  false  aniniou  by  the  upgrowing  of  lateral  folds  (which  have 
not  as  yet  met  in  the  median  line). 

'  For  a  discussion  of  the  origin  and  meaning  of  the  amnion  and  allantois,  see  Balfour,  "  Comparative 
Embryology,"  H.,  p.  256. 


46  CHANGES    IN    THE    UTERUS. 

in  birds,  and  in  some  mammals,  e.g.  ruminants,  this  portion  forms  a  large  sac  which  occupies 
the  greater  part  of  the  cavity  of  the  false  amnion,  and  is  filled  by  fluid  (allantoic  fluid)  in 
which  many  urinary  products  can  be  recognized.  (The  ducts  of  the  embryonic  renal  organs  open 
into  the  pedicle  of  the  allantois.)  But  in  most  mammals  the  development  of  the  hypoblastic 
sac  is  far  less  extensive,  and  in  some,  including  the  human  embryo,  the  allantois  is  mainly 
represented  by  a  large  mesoblastic  outgrowth  carrying  the  allantoic  (umbilical)  vessels  to  the 
chorion.  There  is  also  considerable  variation  in  the  period  at  which  the  allantois  begins  to 
develop.  In  the  human  embryo  it  is  certainly  formed  at  a  very  early  period,  probably  even 
before  the  amnion  is  completed.  In  the  guinea-pig,  also,  it  appears  early,  although  after  the 
amnion.  In  this  animal  it  is  first  developed  as  a  solid  outgrowth  of  mesoblast  which  projects 
from  the  line  of  junction  of  the  hinder  end  of  the  amniotic  bag  with  the  blastoderm,  and 
before  the  formation  of  a  hind  gut  or  of  any  part  of  the  alimentary  tube,  a  hypoblastic 
diverticulum  being  altogether  wanting.  In  the  earliest  human  ova  in  which  the  allantois 
has  been  investigated,  it  is  already  a  tube  of  hypoblast  which  forms  a  direct  prolongation  of 
the  posterior  end  of  the  primitive  alimentary  canal  (fig.  49,  d),  and  is  enclosed  in  a  short 
stalk  of  mesoblast,  by  which  the  posterior  end  of  the  embryo  is  attached  to  the  chorion,  and 
through  which,  by  the  allantoic  (umbilical)  blood-vessels,  the  chorionic  villi  are  freely 
supplied  with  blood.  From  the  attachment  of  this  stalk  to  the  placenta  (chorionic  part), 
the  hinder  extremity  of  the  amnion  is  reflected.  The  stalk  in  question  is  not  the  um- 
bilical cord,  since  it  does  not  include  the  stalk  of  the  yolk  sac  (vitelline  duct),  which  only 
later  becomes  bound  up  with  it.  It  is  termed  by  His  the  abdominal  stalk  (Bauchstiel), 
the  term  allantois  being  by  him  restricted  to  the  hypoblastic  diverticulum.  It  is  further 
considered  by  His  to  be  probable  that  the  human  embryo  never  becomes  completely  detached 
from  the  chorion,  but  that  it  always  retains  its  attachment  to  the  outer  membrane 
of  the  ovum  at  the  hinder  end,  this  abdominal  stalk  being  regarded  as  a  direct  prolongation 
of  the  tail  end  of  the  embryo  (fig.  49).  If  this  should  prove  to  be  the  case,  the 
human  ovum  would  foi-m  an  exception  to  the  usual  rule  of  a  complete  separation  of  embryo 
from  chorion  at  the  formation  of  the  amnion,  and  subsequent  re-attachment  by  outgrowth  of 
allantois. 

Changes  in  the  uterus.     Mode   of  attachment  of  ovum  to   uterus.^ — 

The  mucous,  membrane  of  the  pregnant  uterus  is  known  as  the  cUcidua.  It  is 
thicker  and  more  pulpy  than  in  the  ordinary  non-pregnant  condition,  and  the 
glands  are  longer  in  proportion,  but  it  is  otherwise  of  similar  structure  except  in 
the  part  where  the  placenta  is  about  to  be  formed  ;  here  it  undergoes  important 
modifications.  The  ovum,  wliich  has  been  fertilized  and  has  passed  through  the 
first  stages  of  development  in  the  Fallopian  tube,  although  considerably  larger  than 
the  undeveloped  ovum,  is  still  an  extremely  minute  object  when  it  reaches  the 
uterus.  Here  it  speedily  becomes  imbedded  in  the  soft  and  thickened  mucous 
membrane,  and  this  is  soon  reflected  over  and  completely  encloses  the  ovum,  which 
thus  comes  to  lie  in  a  cavity  within  the  decidua  which  is  altogether  shut  oflP  by  the 
reflected  part  from  the  true  cavity  of  the  uterus.  Different  names  have  been  given 
to  these  parts  of  the  uterine  mucous  membrane  which  immediately  enclose  the  ovum 
to  distinguish  them  from  that  which  lines  the  original  cavity  of  the  uterus.  Thus 
the  layer  of  membrane  which  has  grown  around  the  ovum  is  known  as  the  decidua 
reflexa ;  the  part  where  the  ovum  first  becomes  attached  to  the  uterus  and  where  the 
placenta  is  afterwards  formed,  is  the  decidua  ser_otina,  while  the  membrane  lining 
the  true  cavity  of  the  uterus,  is  termed  deddua  vera  (figs.  50,  51). 

With  the  subsequent  growth  and  consequent  expansion  of  the  ovum  the  enclosing 
decidua  reflexa  expands  also  pari  passu,  encroaching  more  and  more  upon  the  true 
cavity  of  the  uterus  and  coming  into  contact  everywhere  with  the  decidua  vera. 
Eventually  it  blends  entirely  with  the  decidua  vera,  so  that  the  two  layers  are  indis- 
tinguishable and  the  original  cavity  of  the  uterus  is  obliterated  (except  at  the 
cervix  uteri). 

1  The  following  account  of  the  formation  of  the  deciduse  and  of  the  placenta  is  confined  as  much  as 
possible  to  what  has  been  observed  in  the  human  subject.  In  other  mammals  important  variations  in 
the  mode  of  attachment  of  the  ovum  and  in  the  formation  and  structure  of  the  placenta  are  found  to 
Dccur. 


CHAN(JF,s  IN    iiii:  rri-.ins. 


47 


Both  tlu-  .Uri.hiii  v,r:i  iin.l  tlu"  (l.'fi.lua  ivflexa  (.rinimilly  cntaiii  tubular  an.l  somewhat 
tortuous  -lan.ls,  which  wciv  .lisc.v.r.-.l  by  Sharpey.  and  w.t..  by  lii.n  suppos^^l  to  mimstfr.  in 
thofir<t  instance,  both  to  th.-  nutrition  an.l  to  th.-  aitadnn.-nt  ..f  the  ovum,  the  latter  by 
affordin-  depressions  for  the  ehorionic  villi  to  penetrate  into  tin-  substance  of  the  dec.dua.  It 
has  however  since  been  shown  that  these  villi  do  not  .lirectly  pass  into  the  ^'landn,  but 
rather  ten.l  to  become  attache.l  to  the  inter-landular  surface,  an.l  in.leed  at  tin,  de.;idua 
sen.tina,  where  subsequently  th.-  main  attnchment  of  the  chorionic  villi  occurs  the  trland- 
inmina  mav  become  almost  entirely  obliterate.1  before  the  villi  are  here  formed.  But  the 
.n-catlv  enl:ir-e.l  -lands  of  the  decidua  vera  very  probably  furnish  a  secretion  to  af^sist  the 
nourishment  of  the  ovum  previously  to  the  full  establishment  of  the  placenUl  circulation. 


Fig.   50.— DlAGUAMJIATIC    SECTIOS    OF    THE    PREGNANT    HUMAN    UTERUS    AT    THE    SEVENTH    OR 
EIGHTH    WEEK.       (AllcU  ThomSOD.  ) 

c,  c,  opeDings  of  Fallopian  tubes  into  the  cavity  of  the  nterus  :  c',  cervix,  filled  by  a  plug  of  mucijs^ 
thele  te?s^andc'are  placed  within  the  original  ca^^ty  of  the  "terus '•  ^r,  decidua  vera  ;  c/,  de  dinv 
retlexa;  ds,  decidua  serotina  ;  c/.,  choriou  with  its  villi  growuig  into  f  ^^/'^^'f^^Vfinr  in  a  e 
serotina  :  in  the  former  the  ^-illi  are  becoming  atrophied  ;«,  umbd.ca  cord,  the  f;"'^^^'' J*-„^„  "  ^^ 
bloodvessels  within  it;  «/.  allantois  ;  >/,  yolk  sac  (u.nbdi«il  vesicle);  y,  its  stalk,  passing  in  the 
umbilical  cord  and  connected  with  the  intestine  of  the  embryo,  (  ;  am,  amnion. 

The  decidua  undergoes  remarkable  structural  changes  during  the  early  months  ot 
pregnancy,  some  of  these  changes  being  common  to  all  three  parts  of  the  membrane, 


48 


CHANGES    IN    THE    UTERUS, 


whilst  others  are  special  to  that  part  (cl.  serofcina)  which  enters  into  the  construction 
of  the  placenta.     The  following  is  a  brief  account  of  these  changes. ^ 

With  the  supervention  of  pregnancy  the  mucous  membrane  lining  the  uterus 
becomes  thickened  and  the  tubular  glands  become  both  dilated  and  greatly  elongated. 


Fig.  51. — Antero-posterior  section  of  the  gravid 
UTERUS  AND  OVUM  OF  FIVE  WEEKS.  (Semi-clia- 
grammatic. )     (Allen  Thomson. ) 

a,  anterior  ;  p,  jposterior  uterine  wall  ;  m,  mus- 
cular substance  ;  u,  placet!  in  the  cavity  of  the  uterus  ; 
ff,  the  glandular  layer  of  the  decidua  vera  ;  r,  the 
decidua  reflexa  ;  s,  decidua  serotina  ;  c,  cervix  uteri  ; 
ch,  chorion  with  its  villi,  which  are  more  highly  deve- 
loped on  the  placenta]  side  ;  e,  the  embryo  enclosed 
in  the  amnion,  with  the  allantoic  vessels  passing  along 
a  short  allantoic  stalk  into  the  placenta,  and  the 
umbilical  vesicle  lying  free  in  the  space  between 
amnion  and  chorion. 


7?P. 


Fig.  51*. — Diagrammatic  sections  of  the  uterine  mucous  membrane,  showing  the  changes  which 
THE  GLANDS  UNDERGO  WITH  THE  SUPERVENTION  OF  PREGNANCY  (from  Kundrat  and  Eogelmann). 

A,  Diagram  of  the  glands  of  the  non-pregnant  uterus  ;  vi,  muscular  layer  ;  B,  condition  of  the 
glands  at  the  beginning  of  pregnancy  ;  c,  compact  layer  near  free  surface  of  decidua  ;  the  glands  are 
here  somewhat  enlarged  but  not  very  tortuous,  and  the  mucous  membrane  is  rendered  comj)act  by 
hypertrophy  of  the  interglandular  tissue  ;  sj?,  spongy  layer,  containing  the  middle  portion  of  the 
glands  greatly  enlarged  and  tortuous,  producing  a  spongy  condition  in  the  mucous  membrane ;  d,  deepest 
portion  of  the  glands,  elongated  and  tortuous,  but  not  much  enlarged. 

This  thickening  of  the  membrane  and  enlargement  of  the  glands  goes  on  until  the 
fifth  month,  so  that  by  this  time  the  decidua  vera  is  nearly  half  an  inch  in  thickness 
and  its  glands  have  undergone  so  considerable  an  elongation  that  they  now  no  longer 
pass  nearly  straight  through  the  membrane  but  run  in  a  tortuous  manner  from  the 

1  For  a  more  complete  account  of  the  changes  in  the  uterus,  and  of  the  placenta,  the  reader  is 
referred  to  the  list  of  papers  at  the  end  of  this  section,  but  especially  to  the  works  of  Kundrat  and 
Engelmann,  Leopold,  and  Minot,  and  for  the  comparative  structure  of  the  placenta  to  the  classical 
investigations  of  Turner. 


CHANGJ;S    IN    THE    UTERIS. 


40 


inner  surface  to  the  vascular  layei",  so  lliat  a  verl  ical  seel  ion  of  t  lie  iMciiilirano  oxliil)its 
tlieni  cut  (piitc  as  often  ohliijuely  or  trunsvcrsely  as  loiii;iiU(liniilly.  They  are  also 
genenilly  dilated,  bnt  the  dihitation  is  hy  far  most  marked  at  the  mouths  of  the 
glands,  whicii  come  thus  to  have  u  I'uniiel-like  sliajie,  and  in  the  deeper  part  of  the 
membrane,  where  the  dilatations  look  in  sections  like  a  series  of  cavities,  lined  hy 
cubical  or  flattened  epithelium,  ami  separated  from  one  another  by  a  relatively 
small  amount  of  interulandular  substance.  'J'his  gives  a  spongy  api)earan(.-e  to  the 
part  in  question,  and  it  has  been  accordingly  ternied  the  .sLmltiiJi  apo/it/iosiiiii  of 
the  decidua  (tig.  51*,  B,  sp).  The  deepest  part  of  the  glands,  that,  namely,  which  is 
in  contact  with  and  is  imbedded  in  the  sui)erficial  portion  of  the  nniscular  coat,  does 
not  share  in  this  dilatation,  and  its  e])ithelium  also  retains  the  columnar  (character. 
The  i)art  of  each  gland  between  the  funnel-shaped  mouth  and  the  dilatations  aliove 
described,  also  becomes  enlarged,  but  not  to  so  great  an  extent,  the  hypertrophy  of 
the  mucous  membrane  being  here  chiefly  confined  to  the  interglandnlar  tissue,  which 
becomes  filled  with  large  epithelinm-likc  cells  {decidual  cells  of  Friedlander)  and 
with  numerous  and  large  ca])ilhiry  blood-vessels.  This  layer  of  the  decidua  has 
been  termed  the  >^l iJSLifJl^  fP'OL'S"'^ "^^^  '"  (^'o'ltradistinction  to  the  stratum  spongiosum 
external  to  it  (tig.  ol*,  B,  c). 

After  the  fifth  month,  by  which  time  the  great  increase  in  size  of  the  ovum  with 
its  contained  embryo  has  brought  the  decidua  reflcxa  into  close  contact  with  the 
decidua  A'cra,  the  latter  begins  to  undergo  an  atrophic  process,  the  result  to  all 
appearance  of  the  compression  and  distension  to  whicli  it  is  thus  subjected.  Its 
tissue  becomes  thinner  and  less  vascular,  and  both  the  funnel-shaped  mouths  of  the 
glands  and  those  ]iarts  of  the  glands  which  run  through  the  stratum  compactum 
become  gradually  ol)litcrated,  so  that  eventually  hardly  any  trace  remains.     In  the 

Fig.     l)'2. — DlAllK.VMMATIi;    SKOTKiN    TllKCiUGH    THE    UECIDU.K 

AT  THE  EDCE  Hi'  THi;    I'LACEN'TA   (froiii  Kundrat  and 
Engeluiann). 

c,  sp,  111,  as  in  fiy.  51*,  W  ;  d  r,  (](.'ci(]n;i  vera  ;  (/  .'•■,  decidua 
serotina  ;  (/  /■,  dce-idiia  icllcxa. 


stratum  spongiosiun  the  spaces  which  have 
resulted  from  the  dilatation  of  the  gland  tu1)es 
lose  the  lining  epithelium,  and  become  flattened 
out  conformably  to  the  surface,  so  that  they  now 
appear  as  a  layer  of  compressed  lacumu,  sepa- 
rated by  thin  lilirous  tra])ccuky  (tig.  'r2,  s/>.). 

Similar  changes  occur  in  the  decidua  refiexa. 
That  this  is  truly  a  fold  of  mucous  membrane  is 
evidenced  by  the  fact  that  gland  tubes  can  be 
seen  to  open  upon  both  its  surfaces.  These  gland 
tubes  early  become  enlarged,   and    acquire  an 

oblique  or  tortuous  course,  with  dilatations  in  their  deeper  parts,  i.e.,  in  the  middle 
of  the  thickness  of  the  d.  reflexa,  so  as  to  form  here  also  a  sort  of  s}>ongy  tissue. 
But  the  decidua  reflexa  sooner  becomes  ex[)anded  by  the  growing  ovum  into  a 
relatively  thin  meml)rane,  and  tlie  atrophic  changes  in  the  glands  occur  at  an 
earlier  stage,  so  that  by  the  time  that  it  has  coalesced  with  the  decidua  vera  hardly 
any  traces  of  them  can  be  discerned. 

In  the  decidua  serotina  (placental  decidua)  similar  changes  have  been  described 
in  the  glands,  the  final  result  being  the  formation  of  a  spongy  layer,  with  irregular 
clefts  flattened  out  conformalily  to  the  sin'face.  and  from  which  the  ei>itlu'liiun  has 
entirely  disajipeared,  accompanied  by  complete  atrojihy  and  disappearance  of  all  the 
parts  of  the  glands  which  are  superficial  to  this  layer,  the  only  jiortions  which 
remain  nearly  unaltered  licing  the  deepest  parts  of  the  tubes,  which  are   jiartly 

VOL.    r.  -E 


d.s 


50 


CHANGES    IN   THE    UTERUS. 


Fig.  53.— Section  through  a  normal  placenta  of  seven  months  in  situ  (Minot). 

Am,  amnion  ;  CJio,  chorion  ;  Vi,  root  of  a  villus  ;  vi,  sections  of  the  ramifications  of  villi  in  the 
intervillous  spaces,  the  larger  bloodvessels  within  them  are  represented  black :  X»,  deep  layer  ot  tne 
decidua,  showing  flattened  remnants  of  enlarged  glands  in  spongy  stratum  ;  Ve,  uterine  vein  [i  artery) 
opening  out  of  placental  sinus  ;  jUc,  muscular  wall  of  uterus. 


CHANGES    IN    THR    UTERUS. 


51 


imbcdiled  in  tlic  muscular  coat  of  tlie  uterus,  and  rotaiu  thoir  opithclium.  After 
scpai'ation  of  the  placenta  IVcni  tlic  uterine  wall  at  parturition,  the  uterine  mucous 
membrane,  with  its  cpillichuui  and  glands,  becomes  renewed  from  this  deepest  portion 
of  the  decidua  serotina. 

The  most  important  changes  of  structure  occur  in  the  superficial  part  of  the 
placental  decidua,  after  the  disappearance  of  the  glands.  The  exact  manner  in 
which  these  changes  take  place  has  not  been  folhjwed  out,  but  the  ultimate  result  is 
the  replacement  of  the  whole  of  this  poi'tioii  of  the  decidua,  with  the  excejjtion  of  a 


Fig.  54. — Sections  illustkating  the  structure  of  the  placexta  (.Minot). 

A,  vertical  section  througli  the  margin  of  a  placenta  at  full  term  ;  D,  D,  deep  layer  of  decidua ; 
Ft,  chorionic  villi  variously  cut,  their  bloodvessels  injected  ;  Si,  marginal  space  of  the  placenta, 
neai-ly  free  from  villi  ;  ri,  aborted  villi  beyond  the  placenta  ;  Fih,  canalized  fibrine  of  Langhans, 
produced,  according  to  llinot,  by  transformation  of  the  superficial  layer  of  the  chorionic  epiblast. 

B,  decidual  tissue  from  a  i^lacenta  at  full  term  ;  v,  a  bloodvessel  ;  d,  d',  decidual  cells ;  the  latter 
with  several  nuclei. 

comparatively  narrow  basal  kujer  next  to  the  spongy  structure,  into  a  series  of  inter- 
communicating vascular  sinuses,  which  together  constitute  an  immense  flattened 
space  (intervillous  space),  bounded  internally  (toward  the  uterine  wall)  by  the  basal 
layer  just  referred  to,  and  externally  by  the  chorion  ;  also,  according  to  some 
authors,  by  a  thin  layer  of  decidua,  the  suhcJior ionic  membrane  of  Turner,  which  is 
described  as  lying  immediately  under  the  chorion  of  the  o%'um  but  so  intimately 
incorporated  with  it  as  to  be  with  difficulty  demonstrable  as  a  separate  stratum 
except  at  the  edge  of  the  placenta. 

From  the  basal  layer,  partitions  of  fibrous  decidual  tissue  pass  towards  the 
chorionic  surface,  and  serve  to  partially  sub-divide  the  labyrinth  of  vascular  spaces  or 
sinuses  into  a  number  of  locidi  (cotyledons).     Each  of  these  loculi  is  occupied  by  an 

K  2 


52 


ATTACHMENT    OF    THE    OVUM    TO    THE    DECIDUA. 


arborescent  tuft  of  villi  continuous  with  the  fffital  chorion,  and  traversed  by  blood- 
vessels which  are  supplied  from  the  branches  of  the  umbilical  arteries.  These 
blood-vessels  form  a  capillary  loop  in  each  villus,  and  these  capillary  loops  are 
separated  from  the  maternal  blood  in  the  placental  sinuses  not  only  by  the  capillary 
walls  and  the  connective  tissue  of  the  villus,  but  also  by  a  double  layer  of  flattened 
epithelium-like  cells  derived  either  fi-om  the  chorionic  epithelium  (Minot),i  or  from 
the  decidual  tissue,  and,  perhaps,  in  part  representing  an  endothelial  membrane 
belonging  to  the  placental  sinuses,  which,  according  to  Waldeyer,  are  lined  by  endo- 
thelium prolonged  from  that  of  the  uterine  vessels. 

Some  of  the  chorionic  villi  are  attached  (1)  by  comparatively  stout  bands  of 
fibrous  tissue  to  the  basal  stratum  of  the  decidua,  (2)  by  finer  bands  of  similar 
substance  both  to  one  another  and  to  the 

septal  prolongations  of  the  decidua ;  others  amTucn  choncTh 

hang  freely  into  the  placental  sinuses. 
Thes3  sinuses  are  supplied  directly  with 
arterial  blood  from  tortuous  branches  of 
the  uterine  arteries  which  pass  through  the 


e'  r 


Fig.  55. — Diagram  showing  the  tissues  covering 

THE     AaLLI      IN      THE      HUMAN      PLACENTA,     AND 
THEIR     RELATION     TO     THE     DECIDUA    ACCORDING 

TO  Turner. 

F,  fcetal  tissue  ;  M,  raaternal  tissue  ;  d,  blood- 
vessels in  villus  ;  d' ,  blood  in  ijlacsntal  sinus  ; 
e,  layer  of  cells  covering  villus  ;  x.  basement  mem- 
brane covering  villus  (?  endothelium)  continued  from 
uterine  vessels  ;  da,  decidua  serotina ;  t,  tissue 
imiting  villus  to  decidua  ;  ca,  up,  uterine  vessels 
opening  into  sinus. 


Fig.  56. — Diagram  of  the  placenta  (E.  A.  S. ). 

s,  placental  sinus  ;  ds,  decidua  serotina  ;  sp, 
spongy  layer  ;  m,  muscularis ;  a,  v,  uterine 
artery  and  vein  opening  into  placental  sinus. 


spongy  stratum  of  the  decidua  serotina  and  through  the  basal  stratum  of  the 
placenta  to  open  into  the  sinuses  without  the  intervention  of  capillaries.  From 
the  sinuses  veins,  which  run  very  obliquely  through  the  decidua,  carry  off'  the  blood, 
and  eventually  pass  into  the  veins  of  the  muscular  wall.  The  foetal  villi  are  thus 
bathed  by  slowly  flowing  maternal  arterial  blood,  and  respiratory  and  nutritive 
exchanges  may  occur  between  the  two  kinds  of  blood,  but  there  is  no  actual  mixing 

^  Minot  describes  both  the  villi  and  the  sinuses  of  the  placenta  as  bounded  by  the  chorionic  epithelium. 
This  wouM  agree  very  well  with  recent  observations  in  the  bat,  hedgehog,  and  other  mammals,  which 
show  that  these  spaces  are  originally  developed  in  hypertrophied  epiblast  (see  next  page). 


FORMATION    AN'I)    8TRUCTUKK    nl'    IIIM    I'l.ACKN'lA. 


)3 


of  the  two  tluids,  iidi'  is  it  possiMc  U>  inject  tlie  fd'tiis  IVdiM  iIk,'  imillier  iliruiij^ii  the 
placenta,  or  the  mother  IVoiii  the  lu'liis. 

The  question,  whiuh  liiis  hern  iiuiic  Hum  uiiw  niiscil.  wlifilicr  tli<!  intcrvilluiis  spaccB 
nonnally  contain  blood,  may  bo  now  n';.'-anl(Ml  as  settled  in  tlio  aflirniativc.  It  has  lieen 
usually  held  that  they  reiiresent  capillaries  or  veins  of  tlie  decidua,  which  have  become 
dilated  and  fused  to^'etlier  to  sucli  an  extent  as  to  occupy  the  whole  thickness  of  the 
placental  jiart  of  that  membrane,  with  atrophy  of  the  intervening,--  decidual  tissue,  which 
merely  remains  as  a  coverin;;-  to  the  villi.  But  since  the  placental  sinuses  ajjpear  to  be 
bounded  superficially  by  the  chorionic  covering  of  the  ovum,  an<l  it  is  in  most  jjlaces  not 
possible  to  detect  any  decidual  tissue  between  them  and  the  chorion,  it  has  been  conjectured  by 
some  writers  (Kiilliker.  Lani^hans)  that  they  have  become  formed  by  extravasation  of  bloofl  into 
a  space  between  the  ovum  and  the  decidua,  into  and  across  which  sjiace  the  chorionic  villi 
have  ji-rown.  Altliouyh  the  development  of  these  structures  is  insulliciently  known  in  Primates, 
it  has  been  shown  in  various  nuimmals  (Selenka,  Duval,  Ilubrecht,  JIasius)  that  the  first 
attachment  of  the  blastodermic  vesicle  to  the  uterine  wall  ia  effected  by  the  external  layer  of 
the  epiblast,  which  sometimes  splits  off  over  the  embryonic  area  as  a  distinct  layer,  and 
winch,  in  some  animals  (r.//.,  hed;iehoti'),  becomes  greatly  thickened,  and  is  connected  Ijy 
epiblastic  villi  to  the  decidua.  This  external  layer  of  epiblast.  for  which  Ilubrecht  has  pro- 
posed the  general  name  of  troj)]ioh](i>it.  causes  the  absorption  of  the  uteiinc  epithelium  both 
of  tlie  surface  and  of  the  glands  (where  this  epithelium  has  not  previously  been  cast  off) 
and  comes  directly  in  contact  with  the  enlarging  decidual  vessels,  the  endothelium  of  which 
is  actively  proliferating.  AVithin  the  thickened  trophoblast  clefts  now  make  their  appear- 
ance and  are  presently  found  to  be  occupied  by  maternal  blood,  which  is  derived  from  the 
vessels  of  the  adjacent  hypertrophied  decidua.  This  blood  flows  therefore  into  spaces  in  the 
trophoblast.  which  are  only  bounded  by  foetal  epiblast.  and  this  prinuiry  jihirrufal  cirriilati(ni 
may  be  formed  before  any  foetal  blood-vessels  have  reached  the  chorion.  Subsequently,  when 
the  vascular  mesoblastic  villi  become  formed  they  extend  into  theee  spaces,  pushing  before 
them  the  epiblast ;  by  this  layer  they  remain  permanently  covered  and  it  also  lines  the 
enlarged  spaces  into  which  they  have  extended. 

The  placenta  is  composed  of  two  parts,  one  foetal,  composed  of  chorion  with 
its  villi  ;    the  other  maternal,  formed  from  decidua  serotina.      h\  its  completely 


Fig.   r>7. — Transversk  section  op  a  villus  from 

A    PLACENTA    OF    SEVEN    MONTHS  (Miliot). 

Three  blood-vessels  are  seen  within  the  villus, 
imbedded  in  a  jelly-like  connective  tissue  containing 
cells  and  fibres  ;  a,  a,  cell-layer  covering  villus 
(epiblast  according  to  Minot ;  according  to  others 
of  decidual  origin)  ;  /,  a  thickened  portion  of  this 
cell-layer,  which  has  undergone  a  fibrinous  transfor- 
mation (canalised  fibrin). 


developed  condition,  it  is  a  circular  discoid 
mass,  weighing  about  a  pound,  7  or  8  inches 
in  diameter,  thickest  at  the  centre  (1-^  inch), 
and  thinning  away  towards  the  edges, 
which  are  continuous  with  the  compara- 
tively thin  coalesced  decidufB  and  chorion. 
Its  inner  surface  is  smooth  and  concave, 
and  is  closely  covered  by  the  amnion  as  by 

a  serous  membrane  ;  under  this  the  larger  branches  of  the  umbilical  vessels  course 
before  dipping  into  the  substance  of  the  j^lacenta.  From  near  the  centre  of  the 
organ  the  umbilical  cord  passes  off  to  the  foetus.  Its  outer  surface  is  incorporated 
with  the  uterine  wall,  but  when  detached  from  this  by  tearing  through  the  spongy 
tissue  of  which  the  deeper  part  of  the  decidua  is  formed  (as  occurs  in  parturition  at 
the  expulsion  of  the  foetus),  the  outer  surface  appears  ragged  and  irregular,  in 
striking  contrast  to  the  smooth  amnion-covered  inner  surface.  Examined  under 
the  microscope,  the  chorionic  tissue  (villi)  of  the  placenta  is  found  to  be  composed 
of  jelly-like  connective  tissue,  with  brauched  and  anastomosing  cells  (fig.  57)  :  in 
some  parts  of  the  larger  stems  white  fibres  are  seen.     What  remains  of  the  decidual 


54 


THE    PLACENTA. 


tissue  has  a  fibrous  appearance,  with  very  numerous  decidual  cells,  which  frequently 
obscure  the  fibres  (figo  54,  b).    In  and  after  the  fifth  month  of  pregnancy,  a  number 


Fig.   58.  —Front  and  side  views  of  an  early  human  ovum 
FOUR  Ti^ES  THE  NATURAL  SIZE  (from  Eelchert). 

This  ovnm  is  supposed  to  be  of  thirteen  days  after  impreg- 
nation. The  surface  bare  of  villi  is  that  next  the  wall  of  the 
uterus,  showing  at  c,  the  opacity  produced  by  the  thickened 
embryonic  disc.  The  villi  covered  chiefly  the  marginal  parts  of 
the  surface. 


of  large  multinucleated  giant  cells  are  found  scat- 
tered about  in  the  tissue.     They  occur  most  abun- 
dantly  in  the   outermost    layer    of    the    decidua 
serotina,  and  have  been  described  by  Friedlander  and  by  Leopold  as  growing  at  a 
later  sta'o-e  (eio-hth  or  ninth  month)  into  the  veins  which  pass  through  this  layer,  so 


Fig.    59.— YiEW    OF    TEE    INTERIOR    OF    THE    HUMAN    GRAVID   UTERUS    AT    THE    TWENTY-FIFTH   DAT 

(from  Farre  after  Coste). 
a   uterine  wall ;  o,  ovum  with  villous  chorion  ;  dv,  decidua  vera  ;  dr,  decidua  reflexa,  divided  round 
the  margin  of  the  ovum,  and  turned  down  so  as  to  expose  its  pitted  surface,  which  has  been  removea 
from  the  ovum.     The  right  ovary  is  divided,  and  shows  in  section  the  plicated  condition  ot  tJie  eauy 
corpus  luteum. 


SEPARATION    OF   THE   DECIDUA    AT    BiRTH, 


y.^) 


as  to  produce  a  partial  blockage  of  these  veins,  preparatory  to  the  detachment  of  the 
phicenta  from  the  layer. 

The  villi  do  not  at  first  cover  the  wlidle  surface  of  the  ovum,  but  are  deficient  at 
the  embryonic,  and  perhaps  also  at  the  opposite,  pole.  In  the  earliest  human  ovum 
Nvhieh  has  hitherto  heen  described, 
that  of  Reichert  (fig.  bS),  the  villi, 
which  arc  quite  simple,  occur  in  a 
broad  zone  around  the  circumference 
of  the  ovum,  leaving  the  (somewhat 
flattened)  poles  smooth  and  free 
from  villi,  and  on  one  of  these  poles 
a  thickening  of  the  wall  of  the 
vesicle  could  be  detected,  which  was 
probably  the  embryonic  area.  But 
iu  all  other  early  human  ova  whieh 
have  been  noticed,  the  chorion, 
which  is  now  formed  by  the  false 
amnion,  is  covered  with  ramified 
villi  (shaggy  chorion),  and  these  are 
already  vascularized  from  the  allan- 
tois,  and  have  grown  into  the  sub- 
stance of  the  decidua  reflexa  and  the 
decidua  serotina,  the  formation  of 
the  placenta  having  already  begun. 

Separation     of    the     decidua 
uterine  mucous   membrane. — In 


Fig.  60. — Portion  of  an  injected  vilixs  from  a 

PLACENTA  OF    ABODT    FIVE    MONTHS  (Minot). 


at     birth,    and     regeneration    of    the 

parturition,  the  pressure  of  the  contracting 
muscular  walls  upon  the  uterine  contents,  and  especially  npon  the  amniotic  fluid, 
causes  a  bulging  of  the  membranes  (consisting  of  the  combined  deciduas,  the 
chorion,  and  the  amnion)  through  the  os  uteri.  "When  the  membranes  are  ruptured, 
the  amniotic  fluid  first  escapes,  and  subsequently  the  foetus  is  expelled.  With 
further  contraction  of  the  uterus,  the  placenta  becomes  detached  from  the  uterine 
wall,  separating  along  the  plane  of  the  dilated  parts  of  the  glands  (stratum  spongio- 
num  of  the  decidua  serotina),  and  as  it  is  expelled,  the  separation  extends  around  the 
decidua  lining  the  rest  of  the  uterus,  which  appears  in  the  "  after-birth  "  along  with 
the  chorion  and  amnion  as  a  thin  membranous  skirt  to  the  edge  of  the  placenta.  The 
deepest  part  of  the  decidua  containing  the  bases  of  the  uterine  glands  is  everywhere 
left  in  connection  with  the  muscular  tissue,  and  from  these  basal  portions  of  the 
glands,  fii"st  the  whole  of  the  uterine  glands,  and  subsequently  the  lining  epithelium 
of  the  uterus  become  gradually  regenerated. 


KECENT    LITERATURE. 

Allen,  W.,  Omphalo-mesenteric  remains  in  mammals.     Joum.  Anat.  and  Physiol.,  xvii.,  1883. 

Beneden,  Ed.  van,  De  la  fixation  du  hlastocyste  a  la  mitqucuse  uterine  chez  le  murin  (Vesper- 
tilio  murinus).  Bullet,  de  I'Acad.  roy.  de  Belgique,  Ser.  ill..  T.  xv. ;  De  la  formation  ct  de  la  conditu- 
tion  du jilacenta  chcz  le  murin  [Vespcrtilio  murinus).  Bulletins  de  I'Acad.  roy.  de  Belgique,  T.  xv., 
1888. 

Beneden,  E.  v  ,  et  S-alin.,  Rechcrches  sur  la  fornuition  dcs  annexes  foitales  (hezles  mammijercs. 
Arch,  de  biologie,  v.  1S84. 

Bonnet,  R.,  Die  I't.rinmilch  und  Hire  Bedeutung  fiir  die  Frucht.  Beitriija  zur  Biologie,  1882  ; 
Die  Eihciutc  dcs  Pfcrdcs.     Verhandl.  d.  anat.  Gesellschaft,  1889. 

Bumm,  Zur  Kcnntniss  der  Utcroplaccntarjcfasse.  Archiv  f.  Gyniikologie,  xxxv.,  xxxvi'.  ;  Zur 
Anat.  d.  Placenta.     Wiirzburg  Sitzuugsb.,  1889. 

Cadiat,  L' allantoide.     Gaz.  med.  de  Paris,  4  ser.  \'i. ,  1887. 

Cald-well,  W.  H.,  On  the  arranf/c?7ient  of  the  embryonic  memhranes  in  marsupial  animals. 
Quarterly  Journal  of  Microsc.  Science,  xxiv.,  1SS4. 

Colucci,  G.,  Di  alcuni  novi  dati  di  struUura  della  placenta  umana,  1887. 


56  RECENT    LITERATURE. 

Duval,  M.,  Les  placentas  discoides  en  r/eneral,  d  propos  du  plucenta  des  roiy/eiirs.  Comptea 
rendus  de  la  societe  de  biologie,  Ser.  viii.,  T.  v.,  1888  ;  Le placenta  des  ronr/eurs.  Journal  de  I'anat. 
et  de  la  physio!.,  xxv. 

Ercolani,  Cr.  B.,  Nuove  ricerche  di  anatomia  normale  e  patologica  sulla  placenta  dei  mammi- 
feri  e  della  donna.  Mem.  d.  accademia  d.  scienz.  d.  Bologna,  1883.  (French  in  Archives  italiennea 
de  biologie,  iv.) 

Fleischmann,  Ueher  die  erste  Anlage  der  Placenta  bei  den  RauhtMercn.  Sitzb.  der  physik. 
medic.  Soc.  zu  Erlartgen,  1886  ;  Emhryolorjische  Untersachungen.  Heft  i.  V ntersuchungen  iiber 
einheimisehe  jRaubthiere.     Wiesbaden,  1888  ;  Mittelblatt  und  Amnion  der  Katze.     Erlangen,  1888. 

Frommel,  R.,  Entw.  d.  Placenta  v.  Myotiis  murinus.  Wiesbaden,  1888  (Ccntralbl.  f.  Gynak., 
1889). 

Hart,  B.,  The  mechanism  of  the  separation  of  the  jdacenta,  dbc.  Proc.  Roy.  Soc.  of  Edinburgh, 
XV.,  1889. 

Heinz,  R.,  Untersuchungen  iiber  den  Bau  und  die  EntwicTcelimg  der  menschlichen  Placenta, 
Inaug.-Diss.  Breslau,  1888. 

Heinricius,  G-.,  Die  Entwickl.  der  Ilunde-Placenta.  Sitzung.sb.  d.  Berlin.  Akad.,  1889;  Arch.  f. 
mikr.  Anat.  33,  1889. 

HofCmaun,  C.  K.,  Vehcr  das  Amnion  des  zweihldttrigen  Keimes.  Archiv  f.  mikrosk.  Anatomic, 
xxiii.,   1884. 

Hofmeier,  Zur  Anatomic  d.  Placenta.     Wiirzburg  Sitzungsb.,  1889. 

Hubrecht,  The  placentation  of  Erinaceus  Earopceus,  with  remarks  on  the  phylogeny  of  the 
placenta.     Quarterly  Journal  of  Micr.  Science,  1889. 

Kastschenko,  N.,  Das  menschliche  Chorionepithel  und  dessen  Rolle  bei  der  Placenta.  Arch.  f. 
Anat.  u.  Physiol.,  Anat.  Abth.,  1885. 

Keibel,  F.,  Zur  EntivicHungsgesch.  der  menschlichen  Placenta.     Anat.  Anzeiger,  iv.,  1889. 

Klein,  Ueber  die  Entstehung  der  Placenta  marginata.     Arch.  f.  Gynak. .  xxxvi. 

Kundrat  und  Engelmann,  Untersuchungen  iiber  die  Uteruschleinihaut.  Strieker's  med. 
Jahrb..  1873. 

Kuppfer,  C,  Die  Entstehung  der  Allantois  und  die  Gastrvda  der  Wirbelthiere.  Zoologischer 
Anzeiger,  ii. ,  1879  ;  Decidua  und  Ei  des  Menschen  am  Ende  des  ersten  Monats.  Miinchener  Wochen- 
schrift,  1888. 

Langhans,  Die  Losung  der  miitterlichen  Eihdute.  Archiv  fiir  Gynak ol.,  viii.,  1876  ;  Ueber  die 
Zellschicht  des  menschlichen  Chorions.     Henle's  Festgabe,  1882. 

Leopold,  Studien  iiber  die  Uterusschleimhaut  wdlirend  Menstruation,  Schwangerschaft  und 
Wochenbett.  Archiv  f.  Gyniikol.,  xi.,  1877  ;  Ueber  den  Ban  der  Placenta.  Archiv  f.  Gynakologie, 
XXXV.,  1889. 

Lieberkiihn,  N.,  Der  griine  Saum  der  Ilandeplacenta.  Arch,  f,  Anat.  u.  Physiol.,  Anat.  Abth. 
1889. 

Marius,  J.,  De  la  genese  du  placenta  chcz  le  lapin.     Arch,  de  biologie,  ix.,  1889. 

Masquelin  et  Swaen,  Premieres  phases  du  d6veloppement  du  placenta  maternal  chez  le  lapiin. 
Arch,  de  biol.,  i.,  1880. 

Minot,  Uterus  and  embryo.  Journal  of  Morphology,  Vol.  ii.,  1889  (includes  a  tolerably  full 
Bibliography  of  the  subject). 

Nitabuch,  R.,  Beitrdge  zur  Kenntniss  der  menschlichen  Placenta.     Inaug.-Diss.     Bern,  1888. 

Osborn,  H.  F.,  Observations  upon  the  foetal  membranes  of  the  Opossum  and  other  Ma,rsupials. 
Quarterly  Journal  of  Microsc.  Science,  xxiii.,  1883. 

Rohr,  K.,  Die  Beziehungen  d.  miltterl.  Gefdsse  z.  d.  intervilldsen  Jiawnen,  dec.     Virchow's  Archiv, 


Rug-e,  K.,  in  Schroder,  K.,  Der  schwangere  u.  kreissende  Uterus.     Bonn,  1886. 

Ryder,  J.  A.,  The  origin  of  the  Amnion.  American  Naturalist,  xx.,  1886  ;  A  theory  of  the 
oi'igin  of  placental  types,  and  on  certain  vestigiary  structures  in  the  placentce  of  the  mouse,  rat,  and 
field-mouse.     American  Naturalist,  xxi.,  1887. 

Strahl,  H.,  Die  Allantois  von  Lacerta  viriclis.  Sitzungsb.  d.  Marburger  Gesellsch.,  1883;  Ueber 
den  Bau  der  Placenta;  Die  Anlagerung  des  Eies  an  die  Uterusivand.  Arch.  f.  Anat.  u.  Physiol., 
Anat.  Abth.,  1889  ;  Zur  vergleichenden  Anat.  d.  Placenta.  Verhandl.  d.  anat.  Gesellschaft,  1889  ; 
Placenta  von  Putorius  fur 0.     Anat.  Anz.,  iv.,  1889. 

Tafani,  A.,  La  circulation  dans  le  placenta  de  quelques  mammifh-es.  Archives  italiennes  de 
biol.,  viii.,  1887. 

Turner,  "Wm.,  Lectures  on  the  comparative  anatomy  of  the  placenta.  Edinburgh  (Black),  1876  ; 
Some  general  observations  on  the  placenta,  with  especial  reference  to  the  theory  of  evolution.  Joum.  of 
Anat.  and  Physiol.,  xii.,  1877  ;  On  the  placentation  of  the  Apes,  with  a  comparison  of  the  structure  of 
their  placenta  with  that  of  the  human  female,  Phil.  Trans.,  Ixix.,  1879  ;  An  additional  contribution 
to  the  placentation  of  the  Lemurs.     Proceedings  of  the  Royal  Society,  Vol.  xliv.,  1888. 

Waldeyer,  W.,  Ueber  den  Placentarkreislauf  des  Menschen.  Sitzb.  d.  k.  Akad.  d.  Wissensch. 
zu  Berlin,  vi.,  1887  ;  Die  Placenta  von  Inuusnemestrinu-^.  Ibid.,  1889  ;  Menschen-  und  Affcn-placcnta, 
Ai-chiv  f.  mikr.  Anat.,  Bd.  35,  1890. 


DEVELOPMENT    OF   THE    Sl'INAL   roni).  57 

DKVKLOP.MKNT    (»b'    TIIK    SKVKUAL    OllLlANS    <  H'     TIIK     lloDV. 

DEVELOPMENT    OF    THE     NERVOUS    SYSTEM. 

As  has  been  alreaily  ck-scribed,  the  whole  of  the  central  nervous  system  takes  origin 
from  the  thickened  walls  of  a  dorsally  situated  axial  gro(ne,  subsequently  converted 
into  a  canal,  which  runs  forwards  in  front  of  the  primitive  streak,  and  the  anterior 
end  of  which  becomes  enlarged  and  converted  by  constrictions  into  three  successive 
vesicles,  around  which  the  several  ])arts  of  the  brain  are  formed,  and  which  are 
known  as  the  primary  cerebral  vesicles.  The  remainder  of  the  neural  canal  is  of 
nearly  uniform  diameter,  and  its  walls  become  converted  into  the  substance  of  the 
spinal  cord,  while  the  cavity  itself  becomes  eventually  tlic  central  canal  of  the  cord. 
The  walls  of  the  neural  groove  are  of  course  composed  of  epiblast,  and  it  tlierefore 
follows  that  the  whole  structure  of  the  central  nervous  system  is  laid  down  in 
epiblast,  and  consists  in  the  main  of  more  or  less  modified  epiblastic  elements,  except 
where  mesoblastic  tissues  subsequently  penetrate  into  it,  conveying  blood-vessels 
into  its  snbstance.  As  was  shown  by  Balfour,  the  same  is  in  all  probability  true  for 
all  the  nerves  of  the  body,  cranial  and  spinal,  which  either,  as  with  the  fibres  of  the 
anterior  roots  of  the  spinal  nerves,  grow  directly  out  from  the  neural  epiblast,  or,  as 
with  the  fibres  of  the  posterior  roots,  are  formed  and  gi'ow  from  masses  of  epiblast 
cells,  which  are  separated  off  at  the  junction  of  the  neural  and  general  epiblast  to 
form  the  ganglia,  from  which  the  posterior  root  fibres  appear  to  take  origin  (His). 
An  exception  must,  however,  be  recorded  for  the  olfactory  tracts  and  bulbs  and  optic 
tracts  and  nerves,  which,  although  derived  from  the  neural  epiblast,  yet  have  a  different 
mode  of  origin  from  all  other  nerves,  both  cranial  and  spinal,  since  they  arise  not  as 
solid  outgrowths  of  that  epiblast,  but  as  hollow  protrusions  from  the  brain,  which 
only  become  solid  at  a  later  stage  of  development.  We  have  then  to  consider  the 
manner  in  which  are  developed  (1)  the  spinal  cord  ;  (2)  the  several  parts  of  the 
brain  ;  and  (3)  the  spinal  and  cranial  nerves  and  their  ganglia,  as  well  as  the  ganglia 
and  nerves  of  the  so-called  sympathetic  nervous  system. 

DEVELOPMENT    OF    THE    SPINAL   COED. 

Soon  after  the  neural  canal  is  closed  (fig.  32,  p.  31),  it  takes  the  form,  along  the 
greater  part  of  the  length  of  what  is  afterwards  to  become  spinal  cord,  of  a  cleft- 
like cavity,  with  thick  sides,  and  a  relatively  thin  dorsal  and  ventral  boundary  (roof 
and  floor).  The  parietes  of  the  canal  are  wholly  composed  of  long  columnar  epithe- 
lium cells,  whose  free  borders,  which  are  at  first  smooth,  but  later  become  ciliated, 
hne  the  cavity,  and  whose  attached  extremities  rest  upon  a  homogeneous  limiting 
membrane  which  early  makes  its  appearance,  bounding  the  embryonic  cord,  and 
separating  it  from  the  surrounding  structures.  These  cells,  therefore,  extend  at 
first  through  the  whole  thickness  of  the  embiyonic  cord,  and  they  have  the  closely- 
set,  palisade-like  character,  with  the  nuclei  at  different  depths,  such  as  it  is  usual 
to  find  in  long  columnar  epithelium. 

After  a  time,  it  is  found  that  the  cells  (which  have  become  always  longer  with 
the  increasing  thickness  of  the  wall  of  the  neural  canal)  show  a  tendency  to  branch 
and  to  unite  with  the  branches  of  neighbouring  cells.  In  this  way  a  network  or 
spongework  is  produced,  which  extends  throughout  the  gi-eater  part  of  the  thickness 
of  the  embryonic  cord  ;  at  the  same  time  the  inner  parts  of  the  cells  which  immedi- 
ately line  the  canal  retain  their  palisade-like  arrangement,  while  the  external  or 
attached  ends  often  exhibit  a  radiating  disposition,  which  gives  a  characteristic 
radial  character  to  the  external  layer  of  the  reticular  structure.     The  reticulum  is 


58 


DEVELOPMENT    OF    THE    SPINAL    COED. 


termed  myelospongium,  and  the  cells  by  which  it  is  formed  are  spoken  of  as  spongio- 
blasts (His)  (fig.  61). 

Between  the  inner  ends  of  the  columnar  epithelium  cells  or  spongioblasts  there 
is  seen  at  a  comparatively  early  period  (four  and  five  weeks  in  the  human  embryo) 
a  number  of  rounded  cells,  with  a  considerable  amount  of  clear  protoplasm,  forming 


Fig.  61. — Myelospongium  from  spinal  cord  of  three  and  a  half  weeks  human  embryo  (His)  ^i? 
Fig.  62. — Inner  ends  of  spongioblasts  with  germinal  cells,  g,  between  them  ;   from 

SPINAL    cord    of    human    EMBRYO  (His). 

Fig.  63. — Inner  ends  of  spongioblasts  [Sp]  ;  a  gfrminal  cELL(f/)  and  two  transitional 
CELLS  {Tr)  from  spinal  cord  of  human  embryo  (His). 

Fig.  64.—  Three  neuroblasts,  each  with  a  nerve-fibre  process  growing  out  beyond  the 
basement  membrane  of  the  embryonic  spinal  cord  (His). 


an  interrupted  layer  in  this  innermost  zone.  Their  nuclei  are  mostly  in  one  stage 
or  another  of  karyokinesis  (fig.  62).  They  are  termed  by  His  the  germinal  cells,  and 
according  to  him  they  give  origin  to  the  cells  next  to  be  described. 

The  third  kind  of  cell  {neurohlast)  met  with  in  the  cord  of  the  early  embryo  is 
one  with  a  relatively  large  oval  nucleus,  and  little  protoplasm,  but  with  a  taperiug 
protoplasmic  prolongation  directed  outwards  towards  the  surface  of  the  cord.  These 
cells  are  found  in  groups,  at  first  only  in  or  near  the  layer  of  germinal  cells  (fig.  63), 
but  subsequently  in  the  outer  layers  (fig.  64).  The  prolongations  are  the  com- 
mencements of  the  nerve-fibres,  and  they  mostly  converge  either  straight  or  with 
an  arcuate  course  towards  what  will  subsequently  be  the  place  of  exit  of  the  fibres  of 
the  anterior  roots. 

The  outermost  layer  of  the  embryonic  cord  after  the  differentiation  of  the 
various  kinds  of  cells  above  described  is  free  from  nuclei,  and  is  composed  of  the 
partly  reticulated,  partly  radially  arranged  external  or  attached  extremities  of  the 


DEVELOPMENT    OF   THE   SPINAL    CORD. 


59 


spongioblasts.  This  iiuiy  l)e  tiila-n  to  roi>rcsciit  the  white  matter  of  the  cord  at  this 
stage  (all  the  rest  representing  grey  substance)  ;  ijut  there  arc  at  first  no  nerve  fibres 
in  it,  the  only  structures  whieh  can  be  at  all  compared  to  nerve  fibres  being  the 
prolongations  of  the  neuroblasts,  and  these  lie  cither  as  arcuate  fibres  altogether  in 
the  outer  part  of  the  grey  substance,  or  are  passing  out  of  the  cord  as  the  beginnings 
of  the  anterior  roots  from  a  mass  of  nenrol)lasts  which  forms  the  rudiment  of  the 
anterior  cornu  of  grey  matter  (fig.  G5).  This  mass  constitutes  in  the  human  embryo 
of  six  weeks  (fig.  (JG)  the  chief  portion  of  each  half  of  the  cord.  It  forms  a  con- 
siderable projection  which  laterally  almost  reaches  the  surfoce,  but  ventrally  is  sepa- 
rated fn)m  it  liy  a  thickening  of  the  external  or  radial  zone,  due  to  the  appearance  of 
longitudinally  coursing  nerve  fibres  within  it :  this  is  the  begiuninj;  of  the  anterior 


Fig.  65. — Section  of  spinal  cord  of  four  -weeks  human  embryo  (His). 

The  posterior  roots  are  continued  -witliin  the  cord  into  a  small  longitudinal  bundle  which  is  the 
rudiment  of  the  posterior  white  column.  The  anterior  roots  are  formed  by  ihe  convergence  of  the 
processes  of  the  neuroblasts.  The  latter,  along  with  the  elongated  cells  of  the  niyelospongium  compose 
the  grey  matter.  The  external  layer  of  the  cord  is  traversed  by  radiating  fibres  which  are  the  outer  ends 
of  the  spongioblasts.  The  anterior  commissure  is  beginning  to  appear.  This  figure  is  much  more 
magnified  than  the  next  one. 

Fig.  66. — Transverse  section  of  the  cervical  part  of  the  spinal  cord  of  a  hujian 
EMBRYO  of  six  WEEKS  (from  KoUiker).     ^f 

c,  central  canal  ;  e,  its  epithelial  lining  ;  at  e'  (superiorly),  the  original  place  of  closure  of  the  canal  ; 
a,  the  white  substance  of  the  anterior  columns  ;  [/,  grey  substance  of  antero-lateral  horn  ;  ^;,  posterior 
column  ;  ur,  anterior  roots  ;  pr,  jrosterior  roots. 

white  column  (o).  By  this  time,  also,  although  to  a  rather  less  extent,  the  posterior 
white  columns  have,  simultaneously  with  the  posterior  roots,  begun  to  make  their 
appearance  on  either  side  of  the  narrow  dorsal  part  of  the  neural  canal  (;;).  There  is, 
however,  only  a  relatively  thin  layer  of  grey  matter  (neuroblasts)  separating  the 
posterior  white  columns  from  the  palisade-like  lining  of  the  canal,  and  as  yet  no 
sign  of  nerve  fibres  in  the  situation  of  the  lateral  columns,  which  are  only  repre- 
sented by  a  thin  layer  of  the  radial  myelospongium.  The  roof  and  floor  of  the 
canal  are  also  quite  thin  and  undeveloped. 

At  this  period  there  is  still  no  sign  of  either  anterior  or  posterior  (dorsal  or 
ventral)  fissures  of  the  cord.  These  become  formed  as  the  cornua  of  the  grey  matter 
grow  out  from  the  central  mass,  and  as  the  anterior  and  posterior  white  columns 


60 


DEVELOPMENT    OF    THE    SPINAL    COED. 


increase  in  exfcent.  The  anterior  fissure  is  simply  a  cleft  left  between  the  enlarging 
lateral  halves  of  the  cord  ;  the  anterior  commissure  is  formed  across  the  bottom  of 
the  cleft,  which  is  thereby  separated  from  the  central  canal.  As  for  the  posterior 
fissure,  it  is  uncertain  whether  it  is  in  part  formed  from  the  dorsal  portion  of  the 
constricted  canal,  which  has  become  occupied  by  an  ingrowth  of  pia  mater,  and 
converted  into  a  mere  septum  of  connective  tissue,  or  whether  this  fissure  with  its 
connective  tissue  septum  becomes  formed  independently  of  the  central  canal,  which, 
as  the  fissure  extends,  gradually  atrophies  until  it  is  eventually  conveited  into  the 
rudimentary  epithelial  tube  which  is  persistent  during  life. 

In  the  sacral  region  of  birds,  the  central  canal  expands  into  the  rhomboidal  sinus,  and  in 
the  filum  terminale  of  the  human  cord  it  remains  relatively  large.  An  open  enlargement 
analogous  to  the  rhomboidal  sinus  of  birds,  although  relatively  smaller,  has  been  described  by 
Tiedemann  in  a  nine-week  human  foetus. 

The  cord  is  at  first  oblong  oval  in  section,  with  an  angular  depression  in  each 
side  which  serves  to  mark  off  the  situation  of  the  future  posterior  columns  and 
their  coiTesponding  grey  matter  from  the  antero-lateral  region.  These  two  parts  of 
the  lateral  neural  epiblast  may  be  distinguished  as  the  dorso-lateral  (alar)  and  the 
venfcro-lateral  (basal)  laminse  ;   with  the  former,  the  aflFerent  nerve-fibres  become 

Fig.  67. — Brain  and  spinal  coud  exposed  from  behind  in  a  fcetus  of 
THREE  MONTHS  (from  KoUiker). 

/(,  the  hemispheres  ;  m,  the  mesencephalic  vesicle  or  corpora  quadrigemiua  ; 
c,  the  cerebellum  ;  below  this  are  the  medulla  oblongata,  mo,  and  fourth  ven- 
tricle, with  remains  of  the  membrana  obturatoria.  The  spinal  cord,  s,  extends  to 
the  lower  end  of  the  sacral  canal,  and  shows  brachial  and  crural  enlargements. 

connected,  whilst  from  the  latter  the  efferent  fibres  take  origin 
(His),  In  the  human  embryo  of  six  weeks,  they  are  well  marked 
off  from  one  another,  and  their  respective  connections  with  the 
posterior  and  anterior  nerve-roots  are  very  distinct  (fig.  66).  In 
the  upper  part  of  the  cord,  the  lateral  nerve-roots  (spinal  accessory) 
also  arise  from  the  basal  lamina.  The  characteristic  cylindrical 
form  of  the  cord  is  only  attained  with  the  development  of  the 
lateral  columns.  The  cervical  and  lumbar  enlargements  are  mani- 
fest at  the  end  of  the  third  month. 

Up  to  the  fourth  month,  the  cord  and  the  vertebral  canal 
increase  in  length  pari  passu,,  but  the  vertebral  column  then 
begins  to  grow  more  rapidly  than  the  cord,  so  that  by  the  time  of 
birth  the  coccygeal  end  of  the  cord  is  opposite  the  third  lumbar 
vertebra,  while  in  the  adult  its  limit  is  the  lower  end  of  the  first  lumbar.  Along  with 
this  relative  shifting  of  the  cord  and  its  containing  tube,  the  lower  nerve-roots  lose 
their  regular  rectangular  course,  and  become  oblique.  They  alone,  with  the 
filum  terminaU,  occupy  the  lower  end  of  the  neural  canal,  where  they  form  the 
Cauda  equina. 

The  nerve  fibres  of  the  white  columns  are  at  first  entirely  non-medullated,  and 
the  white  substance  has  a  greyish  transparent  appearance.  The  medullary  sheath 
is  not  formed  simultaneously  in  all  parts,  but  appears  at  different  times  in  diflferent 
parts  corresponding  with  the  tracts  of  conduction  ;  the  last  of  these  tracts  to  become 
medullated  are  the  pyramidal  tracts. 

The  membranes  are  formed  from  mesoblast  of  the  protovertebrse,  which  extends 
over  and  under  the  cord,  and  becomes  enclosed  along  with  that  structure  within 
the  developing  vertebral  canal.  The  septa  of  connective  tissue  which  are  seen 
penetrating  into  the  substance  of  the  cord  from  the  pia  mater  grow  in  from  this 
mesoblast,  carrying  blood-vessels  amongst  the  nervous  elements.     The  neuroglia  or 


DEVELOPMENT    OF    THE    BRAIN.  61 

j^enoral  snstentacular  substance  of  both  white  and  grey  matter  is  probably  derived 
from  the  spongioblasts,  and  is  therefore,  like  the  nerve-cells  themselves,  of  epiblastic 
origin. 

DEVEIiOPMENT    OP    THE    BRAIN. 

We  have  already  traced  the  development  of  the  cephalic  part  of  the  neural  tube 
as  far  as  the  formation  of  the  primary  cerebral  vesicles.  Those,  which  are  at  first 
three  in  number  (fig.  40),  become  subdivided  so  as  to  form  five  in  all,  which  may 
be  termed  in  succession  from  before  back,  the  first,  second,  third,  fourth,  and  fifth 
secondary  vesicles.  Of  these  five  parts  the  first  two,  which  represent  the  cerebral  and 
thalamic  parts  of  the  future  brain  (third  ventricle),  are  derived  from  the  first  primary 
vesicle,  and  the  last  two,  the  cerebellar  and  bulbar  parts  (fourth  ventricle),  from  the 
third  primary  vesicle,  while  the  third,  middle,  or  quadrigeminal  part,  represents  the 
undivided  second  primary  vesicle  (Sylvian  aqueduct).  These  relationships,  as  well 
as  the  several  parts  of  the  brain  which  are  eventually  respectively  formed  in  connec- 
tion with  the  vesicles,  are  shown  in  the  subjoined  table. 

'  Anterior  end  of  tMrd  ventricle,  fora- 
mina of   Monro,   lateral   ventri- 
'  First  secondary  vesicle         \       cles,  cerebral  hemispheres,  olfac- 
(^jJvosenceiJhalon)  tory    bulbs  and   tracts,    corpora 

I.  Anterior  primary  vesi-  J  I      striata,  corpus  callosum,  fornix, 

cle  or  fore-brain  | 

„         ,  J  ■  -,  i  Third   ventricle,   optic    nerve    and 

Second  secondary  vesicle  )  ,.  ,.       ,,    ,      •      -^   -. 

V      ,,,    ,  T   7     ^  1       retina,  optic    thalami,  pituitary 

(^thalamencephalon)  ^      and  pineal  bodies. 

II.  Middle  primary  vesicle  /  Third  secondary  vesicle        i  Aqueduct  of  Sylvius,  coi-pora  quad- 
or  mid-brain  \      {mesencephalon)  \       rigemina,  crura  cerebri. 


III.  Posterior  primary  vesi- 
cle or  hind-brain 


( Fourth  secondary  vesicle      /  /  Cerebellum.     Pons. 

{epencephalon')  \  Fourth  ven-  i 

j       tricle  J 

Fifth  secondary  vesicle         \  \  Medulla  oblongata. 

(jnetcncephalon) 


The  first  and  most  striking  change  which  occurs  in  the  primary  brain  is  the 
outgi'owth  on  either  side  of  the  first  primary  vesicle  of  a  hollow  protrusion  {27rim.ary 
optic  vesicle),  which  becomes  developed  eventually  into  optic  nerve  and  retina 
(fig.  68).  The  changes  which  it  undergoes  in  the  formation  of  these  structures  will 
be  considered  when  the  development  of  the  eye  is  dealt  with  ;  suffice  it  for  the 
present  to  say  that  the  free  hollow  communication  (optic  stalk),  which  at  first  exists 
between  the  forebrain  and  optic  vesicle,  becomes  gradually  narrowed  and  at  length 
obliterated,  and  that  as  development  proceeds,  the  connection  of  the  optic  stalk 
becomes  relatively  shifted  backwards,  so  that  when  the  anterior  part  of  the  fore-brain 
is  distinct  from  the  posterior  part,  or  thalamencephalon,  the  optic  vesicle  is  connected 
wholly  Avith  the  latter,  a  relationship  which  is  maintained  permanently,  although 
partially  obscured  afterwards  by  the  later  connection  which  is  formed  between  the 
optic  tract  and  the  mid-brain.  Subsequently  another  pair  of  hollow  outgrowths, 
sprouts  from  the  fore-brain,  and  these  rapidly  extend  forwards,  laterally  and  back- 
wards ;  they  form  the  vesicles  of  the  cerebral  hemispheres.  From  the  roof  of  the  fore- 
brain  (second  vesicle)  a  median  hollow  protrusion  grows  upwards  and  forwards  for  a 
certain  distance  towards  the  vertex,  and  from  the  floor  of  the  same  vesicle  another 
somewhat  similar  protrusion  passes  downwards  and  backwards  towards  the  roof  of 
the  mouth.  The  former  is  the  rudiment  of  the  pineal  gland,  the  latter  of  the 
infimdiiulum,  which  becomes  involved  in  the  formation  of  the  pituitary  body. 

The  principal  parts  of  the  brain  appear  as  thickenings  in  different  parts  of  the 


62 


DEVELOPMENT    OF    THE    BRAIN. 


walls  of  the  vesicles.  Thus  the  corpora  striata  are  formed  in  the  floor  of  the  hemi- 
sphere vesicles,  whilst  the  principal  mass  of  each  hemisphere  is  formed  from  the 
roof  and  sides  (mantle)  of  those  vesicles,  and  the  olfactory  lobes  are  hollow  out- 
Fig.  68. — Fore-part  op  the  embryo  shown  in  fig.  38, 
VIEWED  from  the  DORSAL  SIDE,     'f,     (From  Kblliker.). 

i^,  fore-brain  ;  e,  ocular  vesicles  ;  M',  mid-brain  ;  //,  hind- 
brain  ;  h,  part  of  the  heart  seen  bulging  to  the  right  side  ; 
Voin,  omphalo-mesenteric  or  vitelline  veins  entering  the 
heart  posteriorly  ;  Mr,  medullary  canal,  spinal  part  ; 
p,  protovertebral  somites. 

growths  from  them.  The  cavities  of  the  hemi- 
sphere-vesicles become  the  lateral  ventricles,  and 
the  cavity  of  the  part  of  the  fore-brain  (or  first 
secondary  vesicle)  from  which  they  spring,  forms 
the  anterior  extremity  of  the  third  ventricle. 
The  optic  thalamus  is  formed  by  a  thickening 
of  the  lateral  wall  of  the  second  vesicle,  the 
cavity  of  which  comes  to  be  the  main  part  of 
the  third  ventricle  ;  the  corpora  quadrigemina 
are  thickenings  in  the  roof,  and  the  crura  cerebri 
thickenings  of  the  sides  and  floor  of  the  third 
vesicle,  which  becomes  the  aqueduct  of  Sylvius  ; 
the  cerebellum  and  pons  are  respectively  thick- 
enings of  the  roof  and  floor  and  the  crura  cere- 
belli  of  the  sides  of  the  fourth  vesicle  (anterior 
part  of  hind-brain),  the  cavity  of  which  becomes 
the  anterior  (superior)  part  of  the  fourth  ven- 
tricle ;  and  finally,  the  medulla  oblongata  is 
developed  as  a  thickening  of  the  wall  of  the 
fifth  vesicle,  the  cavity  of  which  expands  from 

the  central  canal  of  the  spinal  cord  to  form  the  calamus  scriptorius  of  the  fourth 

ventricle. 

On  the  other  hand,  certain  parts  of  the  walls  of  the  vesicles  become  thin  and 

greatly  expanded,  and  even  eventually  project  into  the  cavities  as  folds  of  epithelium 

Fig.    69.  —  Outline    of    a    longitudinal 

SECTION     THROUGH      THE      BRAIN      OF      A 

CHICK  OF  TEN  DAYS  (after  Mihalkovics). 

/;,  cerebral  hemisphere  ;  olf,  olfactory  lobe 
and  nerve  ;  st,  corpus  striatum  ;  Iv,  lateral 
ventricle  :  ac,  anterior  commissure ;  It,  laraina 
terminalis;  ope,  optic  commissure ;  pit,  pitui- 
tary gland  ;  inf,  infundibulum  ;  cai,  internal 
carotid  artery  ;  v^,  third  ventricle  ;  dfi,  cho- 
roid jjlexus  of  third  ventricle  ;  2^^''^)  pineal 
gland  ;  bg,  corpora  bigemina  ;  aniv,  anterior 
medullary  velum  ;  below  which  two  last 
references  are  the  aqueduct  of  Sylvius  and 
crura  cerebri;  cbl,  cerebellum;  v^,  fourth 
ha,  basilar  artery  :  ps,  pons  Varolii  ;  cA'*,  choroid  plexus  of  the  fourth  ventricle  ;  obi,  medulla 
;  r,  roof  of  fourth  ventricle. 


ventricle  ; 
oblongata 


covering  ramified  vascular  expansions  of  pia  mater  (choroid  plexuses).  These 
vascular  expansions  occur  along  the  lower  border  of  the  mesial  surface  of  each 
hemisphere-vesicle  (choroid  plexuses  of  lateral  ventricles; ;  along  the  roof  of  the 
second  vesicle  (choroid  plexus  of  third  ventricle),  and  in  the  roof  of  the  fifth  vesicle 
(choroid  plexus  of  fourth  ventricle). 


DEVELOPMENT    OF    SPECIAL    PARTS    OF    THE    BRAIN.  68 

Whilu  tliese  chaiii^cs  are  j^oini,^  on  in  iLs  walls  the  embryonic  brain  docs  not 
remain  straijj:hfc  as  at  first,  with  its  axis  in  a  line  with  tliat  of  tlic  spinal  cord,  but 
undergoes  certain  flexures  (fig.  70),  the  general  result  of  which  is  to  bend  the 
anterior  end  towards  the  ventral  surface.  The  first  of  these  flexures  to  make 
its  appearance  is  a  sharp  bend  opposite  the  base  of  the  mid-brain  and  around  the  ante- 
rior end  of  the  notochord.  The  result  of  this  flexure,  which  produces  a  complete 
doubling  round  of  the  anterior  part  of  the  brain,  is  that  the  mid-brain  is  for  a  time 
the  most  prominent  part  of  the  encci)halon.  liatcr,  the  growth  of  the  cerebral 
vesicles,  and  of  the  thalamencephalon,  brings  these  parts  again  into  prominence,  and 
tends  to  obscure  the  flexure,  which  is,  however,  never  actually  obliterated.  The  second 
cerebral  flexure,  which  is  also  very  sharp  and  well  marked,  occurs  in  the  region  of 
the  hind-brain  (pons  Varolii).  It  is  in  the  opposite  direction  to  the  first  one,  its 
concavity  being  directed  towards  the  dorsum  of  the  embryo,  and  it  produces  the 
appearance  of  a  deep  depression  at  the  part  of  the  brain  where  it  occurs.  The  third 
flexure  is  a  more  gradual  one.  It  occurs  at  the  junction  of  the  hind-brain  with  the 
cord,  the  embryonic  medulla  oblongata  being  bent  ventralwards  fi'om  the  line  of 
direction  of  the  medulla  spinalis. 

The  result  of  these  flexures  is  that  the  axis  of  the  embryonic  brain  takes  a  crook- 
shape,  passing  from  the  end  of  the  spinal  axis  at  first  ventral,  then  dorsal,  and  then 
again  ventral,  finally  bending  sharply  backwards  towards  its  termination  at  the 
foramen  of  Monro. 

The  second  and  third  flexm-es  become  eventually  almost  entirely  obliterated  with 
the  further  growth  of  the  brain. 


FtTBTHER  DETAILS  EEaARDINa   THE   DEVELOPMENT   OF   SPECIAL   PARTS   OF 

THE   BRAIN. 

The  fifth  cerebral  vesicle:  bulbar  vesicle,  or  meteucephalon. — This 
part  of  the  embryonic  brain,  afterwards  to  become  the  medulla  oblongata,  often 
shows  at  its  first  appearance — especially  in  the  chick — a  series  of  slight  constrictions 
(fig.  68),  which  have  by  some  been  taken  to  indicate  a  segmentation  of  the  neural  tube. 
But  even  where  they  occur  they  are  quite  temporary,  and  the  fifth  vesicle  soon  becomes 
a  well  marked  dilatation  opening  out  from  the  anterior  end  of  the  embryonic  spinal 
cord.  Its  wall,  like  that  of  all  the  other  cerebral  vesicles,  is  composed  of  cells 
similar  to  those  of  the  rest  of  the  neural  tube,  and  the  histogenetic  changes  which 
occur  to  form  the  nervous  tissue  are  also  entirely  similar. 

Sections  across  this  part  of  the  neural  tube  are  of  a  compressed  oval  outline  in 
the  lower  part  (fig.  71,  A,'b),  but  in  the  upper  part,  which  afterwards  becomes  the 
lower  part  of  the  fourth  ventricle,  the  thinning  out  and  lateral  expansion  of  the 
dorsal  wall  of  the  tube  gives  to  sections  of  this  and  the  next  (fourth)  vesicle  the 
shape  of  an  irregular  triangle,  or  shield,  the  base  of  the  triangle  being  directed 
towards  the  dorsum  (roof)  and  the  sides  bent  more  or  less  sharply  inwards  about 
their  middle  to  unite  with  one  another  ventrally  at  the  apex  of  the  triangle  (figs. 
72,  73).  This  bend  serves  to  mark  a  division  of  each  side  of  the  tube  into 
two  parts,  a  dorso-lateral  and  a  ventro-lateral,  which  correspond,  both  in  their 
situation  and  in  their  relationship  to  afferent  and  efferent  nerves,  with  the  alar 
and  basal  laminse  of  the  embryonic  cord  (p.  GO),  with  which  they  are  in  fact 
continuous.  The  thinning  out  and  lateral  expansion  of  the  roof  in  the  region 
of  the  fourth  ventricle  tends  to  open  up  the  angle  which  the  ventral  laminiB 
form  with  one  another,  and  to  throw  the  dorsal  laminte  more  to  the  side,  so  that 
what  were  previously  the  lateral  boundaries  of  the  neural  tube  come  to  occupy  the 
so-called  floor  of  the  fourth  ventricle,  and  since  in  this  region  the  roof  becomes 


NK 


i^^ji^Kiw^%r\  y 


Fig.    70. PllOFILE 


iEVEKAL    STAGES, 


i'lEWS    OF    THE    BRAIN    OF    HUMAN    EMBRYOS    AT    THREE 
KECON.STRUCTBD    FROM    SECTIONS  (His). 

A.  Brain  of  an  emlnyo  of  about  15  clays  (the  embryo  itself  is  shown  in  fig.  ]17)  magnified  35 
diameters. 

B.  Brain  of  an  embryo  about  three  and  a  half  weeks  old.      The  optic  vesicle  has  been  cut  away. 

C.  Brain  of  an  embryo  about  seven  and  a  half  weeks  old.     The  optic  stalk  is  cut  through. 

A,  optic  vesicle  ;  //,  vesicle  of  cerebral  hemisphere,  first  secondary  vesicle  ;  Z,  thalamencephalon, 
second  secondary  vesicle  ;  M,  mid-brain ;  /,  isthmus  between  mid-  and  hind-brain  ;  HK  fourth 
secondary  vesicle  ;  TV,  fifth  secondary  vesicle  ;  Gh,  otic  vesicle  ;  Rf,  fourth  ventricle  ;  Nl\  neck 
curvature  ;  Br,  pons  curvature  ;  Pm,  mammilary  process  ;  Tr,  infundibulum  ;  Hp  (in  B),  outline  of 
Jiypophysis-fold  of  buccal  epi blast  ;  111,  olfactory  lobe.  In  C  the  basilar  artery  is  represented  along  its 
whole  course. 


DEVELOPMENT    ol'^    TIIK    .MKDI   I.LA    oULo.Nd A  TA. 


65 


reduced  to  a  thin  laycT  of  tlaitciied  t']iiLlii'Iiiiiii,  tlie  substance  of  this  part  of  the 
medulla  ul)l()n,u;ata  is  wholly  lunnrd  liya  thickcniii}^  of  tin-  shifted  lateral  buiiiidarics. 
In  these,  the  lieiid  luarkiiij;-  the  disiiiictioii  between  the  ventral  anddnrsal  laniiiuc — 
now  by  change  of  jiosition  mesial  and  external — eontiiuies  to  l)e  evident,  and  is  in 
fact  recognizable  even  in  sections  of  the  fnlly-develoijcd  brain. 

Of  the  longitudinal  columns  of  the  medulla  oblongata  the  restiform  bodies  first  bc-come 
prominent  (third  month  in  the  human  embryo).  Tlie  (ant(inor)  pyramids  arc  oljvious  in  the  fifth 
month,  and  the  olivary  tubercle  about  the  sixtli.     liut  belore  any  of   these,  and  indeed  with 


Fig.  71. — Sections  across  the  region  op  thb  calamus  scriptorius  of  the  br^in  represented 

IN  FIG.  70,  A.     (His.) 

A,  region  of  the  glossopharyngeal  ganglion. 

B,  of  the  auditory-facial  ganglion. 

Fig.  72. — Sections  across  the  fourth  ventricle  op  a  somewhat  older  embryo  (His.) 

A,  section  taken  through  the  lower  part. 

B,  across  the  widest  ijaii-  (trigeminus  region). 

C,  through  upper  part  (cerebellar  region). 

r,  roof  of  neural  canal  :  al,  alar  lamina  ;  hi,  basal  lamina  ;  v,  ventral  border. 

Fig.  73. — Sections  across  the  lower  half  of  the  fourth  ventricle  of  a   still  older  embryo, 

SHOWING    GRADUAL    OPENING    OUT    OP    THE    NEURAL    CANAL    AND     THE    COMMENCING    FOLDING    OVER    OK 
THE    ALAR    LAMINA  (at/). 

1%  ventral  border  ;  t,  tiEnia  ;  ot,  otic  vesicle  ;  rl,  recessus  labyrinthi. 

In  the  succeeding  stage  (not  here  represented)  the  angle  at  c  has  almost  disappeared,  the  fold  /  has 
extended  over  the  alar  lamina,  and  the  two  thickened  halves  are  in  the  same  horizoutal  plane,  covered 
by  a  gi-eatly  expanded  and  thinned  out  roof. 

VOL.   I.  F 


66 


OEEEBELLAK    VESICLE. 


the  earliest  appearance  of  the  nerve  roots,  the  white  bundles — not  yet  medullated,  how- 
ever-— which  are  known  as  the  ascending  root  of  the  fifth,  and  the  ascending  root  of  the  vas'us 
and  glossopharyngeal  (solitary  bundle)  begin  to  make  their  appearance,  both  being  at  first  on 
the  surface  of  the  medulla.  They  gradually,  however,  become  covered  in  by  a  folding  over  of 
the  dorsal  part  of  the  alar  lamina,  and  thus  come  later  to  lie  imbedded  in  the  substance  of 
each  lateral  half  of  the  medulla.  This  fold  is  shown  in  its  commencement  in  fig.  73,  A  and 
B,/.  According  to  His  the  bundles  grow  downwards  towards  the  spinal  cord  from  the  places 
of  entrance  of  the  corresponding  nerve  roots,  emerging  from  the  ganglia,  as  in  the  case  of  the 
posterior  spinal  roots ;  and  after  entering  the  medulla  grow  gradually  along  the  course  of  the 
future  so-called  ascending  roots,  so  that  the  latter  are  at  first  visible  only  in  sections  taken 
near  the  places  of  entrance  of  the  nerve  roots  into  the  medulla. 

The  fourth  cerebral  vesicle :  cerebellar  vesicle,  or  epencephalon. — The 

constriction,  which  is  at  first  obvions  between  this  and  the  fifth  vesicle,  does  not 
long  persist,  so  that  the  two  together  form  a  long  boat-shaped  cavity  which  becomes- 

Fig.  7-4. —Median  section  through 

THE     BRAIN     OF     A     TWO     AND     A, 
HALF      MONTHS      FCETUS.  (His.) 

Magnified  5  diameters. 

The  mesial  surface  of  the  left 
cerebral  hemisphere  is  seen  in  the 
upper  and  right  hand  part  of  the 
figure  ;  the  large  cavity  of  the  third 
ventricle  is  bounded  above  and  in 
front  bj'  a  thin  lamina  ;  below  is  seen 
the  infundibukim  and  pituitai-y  body. 
Filling  the  upper  part  of  the  cavity 
is  the  thalamus  opticus  ;  in  front  and 
below  this  is  the  slit-like  foramen  of 
Monro.  Behind  the  thalamus  is  seen, 
another  slit-like  opening  which  leads 
into  the  still  hollow  external  geni- 
culate  body, 

olf,  olfactory  lobe  ;  f,  pituitary 
body  ;  c  q.,  corpora  quadrigeraina  ; 
cb,  cerebellum;  m.o.,  medulla  ob- 
longata. 

the  fourth  vmtricU.  As  in 
that  part  of  this  cavity  which 
has  already  been  described  with  the  fifth  vesicle,  the  roof  inferioiiy  becomes  greatly 
thinned  and  expanded.  Superiorly  the  tube  becomes  gradually  more  contracted  and 
the  roof  thicker,  this  thiGkening  being  the  rudiment  of  the  cerebellum  and  of  the 
valve  of  Vieussens  (fig.  74).  In  the  meanwhile  a  considerable  thickening  of  the 
lateral  boundaries,  which,  as  in  the  medulla  oblongata,  have  been  thrown  outwards 
by  the  roof  expansion,  occurs,  and  from  this  the  substance  of  the  fons  is  gradually 
formed. 

The  dorsal  and  ventral  laminge  of  the  lateral  walls  are  still  evident  in  this  part  of 
the  embryonic  brain.  With  the  former,  the  sensory  fibres  of  the  fifth  nerve  are 
immediately  connected  ;  with  the  latter,  the  motor  fibres  of  the  fifth  and  also  the 
sixth  and  seventh  nerves. 

In  the  human  embryo  the  cerebellum  is  seen  as  early  as  the  second  month, 
forming  a  thin  plate  arching  over  the  anterior  part  of  this  vesicle  (fig.  74),  From 
this  plate,  which  enlarges  only  gradually,  is  formed  the  middle  lobe  ;  later  the  lateral 
lobes  grow  out  at  the  sides.  The  cerebellar  surface  is  at  first  smooth,  but  a  sub- 
division into  the  subordinate  lobes  occurs  in  the  fifth  month,  and  the  folia  appear 
about  the  sixth.  In  the  seventh  month  all  the  parts  of  the  organ,  except  the 
amygdalte,  are  formed. 

Of  the  cerebellar  peduncles,  the  inferior  appear  in  the  third  month,  the  middle  in 
the  fourth,  and  the  superior  in  the  fifth.  The  transverse  fibres  of  the  pons  develop 
pari  passu  with  the  lateral  lobes,  appearing  about  the  fourth  month. 


THE    Sl'X'OND    AM»     IIIIKI)    (  KltKliKA  I.    N'ESICLKS. 


f!7 


The  third  cerebral  vesicle  :  mesencephalon:  mid-brain. — Ii:  this  rc;;iou 
no  cxpiiiisiiui  of  tlio  Vesicle  \vi til  tliimiint;  nrtlie  loof  (jcciirs,  as  in  the  others,  ljiit,uii 
the  contrary,  the  roof  uiuleruoes  coiisideral)le  thickening  (tig.  74).  About  the  third 
month,  this  thickening  becomes  separated  into  two  by  a  median  groove.  These  cor- 
respond with  the  corpora  bigcmina  of  lower  vertel)rates  ;  it  is  only  in  mammals  that 
they  become  further  subdivided  by  a  transverse  furrow.  This  appears  in  nian  about 
the  fifth  month,  and  the  eminences,  which  are  at  first  large  in  proportion  to  the  size 


¥i<2 


7i>.  —  F(KTAL     IIR.MN- 

TiiniD  MONTH.     (Hi 


The  brain  is  repiesenteil  in 
profile,  but  tlie  external  ".yall  of 
the  right  hemisphere  has  been 
removed  to  show  the  interior  of 
the  lateral  ventricle  with  tlie  cor- 
pus striatum  curving  round  the 
bend  of  the  fossa  of  Sylvius.  The 
curved  projections  above  tiie  cor- 
pus striatum  are  infoldings  of  the 
mesial  wall  of  the  hemisphere 
vesicle.  The  lettering  as  in  tig.  74. 

of  the  brain,  thus  become 
the  corpora  quailrujemina. 

The  fibres  of  the  third 
nerve  originate  in  the  ven- 
tral lamina  of  this  part  of 
the  neural  tube :  the  teg- 
meniinn  and  rrusta  become  "i-o- 
formed  &s  thickenings  along 
the  same  lamina  :  the  vesi- 
cle itself  becomes  the  aqtie- 
duct  of  Sylvius. 

In  the  constriction  between  the  third  and  fourth  vesicles  {isthmus  of  His)  the 
fourth  nerve  takes  origin  in  the  ventral  plate  of  the  neural  tube. 

The  second  cerebral  vesicle  (thalamencephalon). — It  is  from  this  part 
of  the  neural  tube  that  the  prir)iar]j  optic  resides  are  developed  in  the  earliest 
period,  and  they  are  for  some  time  in  free  communication  with  its  cavity  along  the 
hollow  optic  stalks.  But  with  the  formation  of  the  optic  nerves  and  optic  tracts, 
the  stalks  become  solid,  and  are.  moreover,  connected  posteriorly  with  the  mid-brain 
by  a  prolongation  backwards  of  the  tracts.  The  optic  thalamus  of  each  side  is 
formed  by  a  thickening  of  the  lateral  wall  of  the  vesicle  (figs.  74,  78).  The  interval 
between  the  thalami  forms  the  cavity  of  the  third  ventricle.  Across  it  the  gi'ey 
commissure  subsequently  stretches.  The  floor  becomes  prolonged  downwards  into 
the  infundibulum,  and  takes  part  in  the  formation  of  iha  pituitary  hody  (figs.  69,  74). 
The  roof,  on  the  other  hand,  becomes  like  that  of  the  fifth  vesicle,  thin  and  expanded, 
and  remains  as  a  single  layer  of  flat  epithelium  cells  inflected  into  the  ventricle  and 
subsequently  occupied  by  vascular  growtlis  of  pia  mater  (choroid  plexus  of  third 
ventricle,  fig.  69,  ch^).  But  at  the  posterior  part  of  the  roof  there  is  a  transverse 
thickening  to  form  the  posterior  commissure,  and  in  front  of  this  the  roof  grows 
upwards  and  forwards,  but  subsequently  backwards  (in  man)  as  a  hollow  median 
process  to  form  the  jnneal  yland  (epiphysis  cerebri).  The  median  process  soon  takes 
on  a  tubular  shape  (fig.  6'J,  pin),  and,  after  a  time,  becomes  branched,  and  forms  a 
number  of  tubidar  follicles  lined  by  ciliated  epithelium,  and  invested  by  vascular  pia 
mater.  These  follicles  tend,  in  man  and  mammals,  as  development  proceeds,  to 
become  solid  and  occupied  by  calcareous  deposit.     But  in  some  reptiles  the  pineal 

F  2 


68 


THE    FIRST    CEREBRAL    VESICLE. 


tube  remains  single,  and  appears  as  the  long  stalk — partly  hollow,  and  partly  solid 
—of  a  rudimentary  median  or  parietal  eye,  which  occupies  an  aperture  in  the  middle 
line  of  the  skull  (de  Graaf,  Baldwin  Spencer).  The  pituitary  body  (hypophysis 
cerebri)  is  chiefly  formed  by  a  diverticulum  of  the  buccal  epiblast  (diverticulum  of 
Rathke)  which  grows  upwards  towards  the  base  of  the  second  cerebral  vesicle,  and 
dilates  into  a  flask-shaped  expansion  which  is  at  first  simple,  but  subsequently  grows 
out  to  form  a  small  mass  of  epithehal  tubes,  the  lumen  of  which  becomes  eventually 
obliterated.  Against  the  posterior  wall  of  this  flask-shaped  dilatation  the  infun- 
dibulum  grows  down  from  the  floor  of  the  second  vesicle,  and  its  extremity  becomes 


Fig.  76. — Median  sagittal  section  of  the  head  in  early  embryos  of  thk  rabbit.     Magnified. 

(From  Milialkovics. ) 

A.  From  an  embryo  five  millimetres  long. 

B.  From  an  embryo  six  millimetres  long. 

In  A,  the  faucial  opening  is  still  closed  ;  in  B,  the  septum  is  perforated  at  /;  c,  anterior  cerebral 
vesicle  ;  mc,  mesencephalon  ;  mo,  medulla  oblongata  ;  m,  medullary  ei^iblast  ;  ?/  (in  B),  infundibulum  ; 
spc,  spheno-ethmoidal,  be,  sphenoidal,  and  spo,  spheno-occipital  parts  of  the  l^asis  cranii  ;  i,  foregut ; 
ch,  notochord  ;  py,  buccal  pituitary  involution  ;  am,  amnion  :  h,  heart. 

Fig.  77. — Median  sagittal  section  of  the  infundibulum  and  pituitary  diverticulum  in  a 

RABBIT    EMBRYO,    AFTER    THE   OPENING   OF   THE   FAUCES.       (From  MihalkovicS. ) 

he,  basis  cranii  with  basilar  artery ;  if,  infundibulum ;  tha,  floor  of  thalamencephalon  ;  py,  pituitaiy 
diverticulum,  now  closed  ;  p',  stalk  of  original  communication  with  the  mouth  ;  pJi,  pharynx  ; 
ch,  notochord  in  the  spheno-occipital  part  of  the  cranial  basis. 

intimately  connected  with  the  dilatation,  but  without  communicating  with  its 
cavity,  although  bound  up  together  by  the  same  vascular  connective  tissue.  In 
connection  with  this  extension  of  the  infundibulum,  nerve  cells  and  fibres  become 
formed  ;  in  lower  vertebrates  they  persist  and  retain  their  connection  with  the 
brain.  The  notochord  extends  in  the  basis  cranii  as  far  as  the  pituitary  body.  Just 
before  reaching  this,  it  bends  ventralwards  towards  Eathke's  diverticulum,  and  here 
blends  with  the  buccal  epiblast  (Bonnet). 

Dohrn  has  shovs^n  that  in  Petromyzon  the  hypophysis  developes  as  a  separate  median 
diverticulum  of  the  external  epiblast  which  is  formed  between  the  nasal  pit  in  front  and  the 
buccal  invagination  (stomodasum)  behind,  and  grows  straight  backwards  as  a  canal  of  some 
length  towards  the  point  of  the  notochord.  where  follicles  develope  from  it  and  become  con- 
nected with  the  infundibulum.  Later,  its  orifice  is  found  to  open  in  common  with  that  of  the 
nasal  pit. 

The  first  cerebral  vesicle :  prosencephalon. — This  is  represented  by  the 
common  point  at  the  front  of  the  third  ventricle,  whence  the  hemisphere  vesicles 
diverge  through  the  foramina  of  Monro,  and  by  these  vesicles  themselves.  The 
original  vesicle  is  therefore  relatively  small,  although  its  lateral  outgrowths  form  by 


Till':    FIRST    OEKEBRAL    VESICLK. 


GO 


far  till'  largest  pcirt lull  of  tlu'  hraiii  in  hi j^hcr  vertebrates.     Tlic  corpora  striata 
appear  as  tliickfiiiiijis  (if   tlif  tiour  of  tlifs  hemisphere  vesicles,  and  oiitsitlc  them  tLu 


Fig.  78. — Three  sections  through  the  foke-brain  of  a  four  and  a  half  weeks  embryo.    (HLs.) 

A.  Througli  the  lower  .interior  part  of  tbe  fore-braiu  ;  S,  fal.\  :  Sf.  fold  of  roof  pa.ssing  below  falx 
towards  the  third  ventricle  ;  Bf,  fold  forming  the  sulcu.s  amuionis  ;  v. HI,  k.I!/.,  anterior  and  posterior 
parts  of  olfactory  lobe  ;  Cs,  corpus  striatum  ;  0.  JF,  groove  continuous  with  optic  stalk  ;  P.s,  pai"s 
subthalamica  ;  7'.c,  tuber  cinereum. 

B.  Section  a  little  further  back.  Sf  in  replaced  by  a  less  prominent  but  broader  fold  of  the  roof. 
Ad,  which  subsequently  receives  the  choroidal  vessels,  and  is  therefore  the  choroidal  fold  ;  i/.y, 
hemisphere  vesicle  ;  Th,  thalamus  ;  S.M,  sulcus  of  Monro,  below  ami  behind  the  thalamus. 

C.  Still  further  back  :  ..If^  choroidal  fold  here  projecting  into  lateral  ventricles,  but  still  free  from 
mesoblast  and  bloodvessels  ;   Ma,  mammillary  tubercle.     The  other  lettering  as  before. 


70 


THE    FIRST    CEREBKAL    VESICLE. 


grey  and  white  matter  of  the  island  of  Reil  becomes  differeutiated.  The  rest  of  the 
wall  of  the  hemisphere  vesicle  {imniJe  of  Eeichert),  although  remaining  for  a  time 
thinner  than  the  floor,  eventually  thickens  to  form  the  whole  of  the  grey  and  white 
matter  of  the  hemisphere,  except  along  the  line  where  the  mesoblast,  which  is  to 
form  the  choroid  plexus  of  the  lateral  ventricle,  passes  into  the  choroidal  fold  of  the 
neural  epil^last,  which  becomes  thinned  out  over  the  invading  mesoblast  and  con- 
verted into  the  epithelial  covering  of  the  plexus. 

The  growth  of  the  hemispheres  takes  place  gradually.     They  extend  at  first  some- 
what forwards  and  upwards,  separated  by  a  thin  layer  of  mesoblast  which  forms  the 


f 


h 


h 


Fig.    79.  —  Tka^syerse     sectio>-s 

THROrcH      THE      BRAIN        OF       A 
sheep's    E3IBRT0    OF    2 '7    CM.    IN 

LE^'GTH.     (From  Kolliker. ) 

In  A  the  section  passes  through 
the  foramina  of  !Monro,  in  B  through 
the  third  and  lateral  ventricles  some- 
what further  back,  st,  corpus  stria- 
tum ;  th,  optic  thalamus  ;  t,  third 
ventricle  ;  r.  c',  rudiment  of  internal 
capsule  and  corona  radiata ;  ',  lateral 
ventricle  with  choroid  jilexus,  pi ; 
h,  hippocampus  major  ;  /,  primitive 
falx  ;  a,  orbito-sphenoid  ;  sa,  pre- 
sphenoid  :  p,  pharynx ;  ch,  chiasma  ; 
o,  optic  nerve  ;  m,  m,  foramina  of 
]\Ionro  ;  to,  optic  tract ;  mk,  ^Meckel's 
cartilage. 

falx,  but  soon  begin  to  pass 
jDrogressively  backwards,  so 
that  by  the  end  of  the  third 
month  they  have  covered  the 
region  of  the  optic  thalami,  by 
the  fourth  month  they  have 
reached  the  corpora  quadri- 
gemina,  and  by  the  sixth  they 
cover  not  only  the  corpora 
quadrigemina,  but  also  a  great 
part  of  the  cerebellum,  iDro- 
jecting  even  beyond  this  by 
the  end  of  the  seventh  month. 
In  front,  and  for  some  dis- 
tance backward  over  the  roof 
of  the  third  ventricle,  wdiere 
the  vesicles  are  not  separated 
by  the  falx,  their  mesial  sur- 
faces come  into  contact,  and, 
during  the  third  month,  partly 
grow  together,  but  in  such  a 
manner  as  to  leave  anteriorly 
just  in  front  of  the  third 
ventricle,  a  triangular  ares 
free  in  the  middle,  but  completely  surrounded  at  its  periphery  by  the  uuited  parts. 
Thus  is  formed  the  cavity  which  is  known  as  the  ffth  ventricle  or  ventricle  of  the 
septum  Jucuhim,  which  at  no  time  has  any  communication  with  the  vesicles  of  the 
cerebral  hemispheres,  nor  with  any  other  of  the  cerebral  vesicles. 


st- 


( 


^-A 


THK    FlIIST    CHJlKr.KAL    VRSICLE 


71 


The  eoiiunissures  of  the  cerel)ral  hemispheres  are  also  formed  in  tlie  united  portion 
of  the  mesial  walls  of  the  vt'sicles  ;  the  anterior  is  the  earliest  to  apjxjai-,  thus 
coincidinii-  with  the  early  appearance  of  the  corpora  striata,  which  it  unites  in  trout. 
The  anterior  part  of  the  fornix,  with  its  pillars,  and  the  (•ori)Us  albicans  (which  is 
at  first  single  and  median)  are  next  formed,  followed  at  a  later  ]>eriod  by  the 
jiosterior  pillars,  which  are  seen  runnins:  backwards  on  each  side  into  the  comu 
ummonis  as  soon  as  this  structure  becomes  distinct.  The  corjuis  callosum  is  the 
last  of  the  commissures  to  be  formed,  its  anterior  part  apjjcars  first,  but  as  the 
hemispheres  extend  l)ackwards  the  formation  of  the  commissure  accompanies  the 
l)ackward  extension. 

The  olfactory  lobes  arc  formed  as  hollow  outgrowths  fi'om  the  lower  and 
lateral  parts  of  the  hemisphere  vesicles  (figs.  70,  7-4).  In  man  thoy  soon  show  a 
division  into  two  parts,  an  anterior  and  posterior  ;  of  which  the  latter  remains  in 
close  connection  with  the  hemisphere  vesicle,  while  the  anterior  grows  out  towai'ds 


Fig.  80. — The  surface  of  the  F(£Tal  brain  at  six  months.     (K.  "Wagner.) 
This  figure  shows  the  foi-mation  of  the  principal  fissures.     A,  from  above  ;  B,  from  the  left  side. 
F,  frontal  lobe  ;  P,  parietal  ;  0,  occipital ;   T,   temporal  ;  a,  a,  a,  slight  appearance  of  sulci  in  the 
frontal  lobe  ;  .>•■,  Sylvian  fissure  :  *•',  its  anterior  division  ;  witliin  it,  C,  the  central  lobe  ;  j;  Rolandic 
sulcus  ;  p,  parieto-occipital  fissure. 

Fig.  81. — View  of  the  inner  surface  of  the 

RIGHT  half  of  THE   F(I:TAL  BRAIN    OF  ABOUT 

SIX  MONTHS.     (Reichert.) 

F,  frontal  lobe  ;  P.  parietal ;  0,  occipital ; 
T,  temporal  ;  /,  olfactory  bulb ;  //,  optic 
nerve  ;  /}),  callo.so- marginal  fissure  ;  ii,p',  parts 
•of  the  pai'ieto- occipital  fissure ;  h,  calcarine 
fissure ;  g,  g,  gyrus  fornicatus ;  c,  c,  coi-pus 
•cftllosum  ;  s,  septum  hicidum  ;  /,  placed  be- 
tween the  middle  conimissure  and  the  foramen 
of  ^Innro  ;  v,  in  the  upper  part  of  the  third 
ventricle  ;  v',  in  the  back  part  of  the  third 
ventricle  ;  v",  in  the  lower  part  of  the  third 
ventricle  above  the  infumliljulum  ;  r,  rec^ssus 
pinealis  ;  pr,  pons  Varolii  ;    C'c.  cerebelhmi. 

the  olfactory  area  of  the  external  epiblast.  After  the  first  month  these  lobes  are 
relatively  small  in  size  and  their  cavities  become  gradually  obliterated,  but  in  some 
animals,  as  in  the  horse,  they  are  large,  and  their  cavities  are  permanently  in 
communication  with  the  anterior  eornua  of  the  lateral  ventricles.  Their  fiuther 
development  is  given  subsequently  (p.  7!»). 

Formation  of  the  fissures  and  convolutions.  —  The  enlargement  of  the 
cranium  does  not  always  keep  pace  with  the  growth  m  extent  of  the  walls  of  the 


t.  ^l- 


THE    FIRST    CEREBRAL    VESICLE. 


hemisphere  vesicles,  so  that  it  happens  that  the  former  are  thrown  into  folclt 
separated  by  sulci,  and  the  surface  loses  its  smooth  appearance.  Such  relatively 
Tapid  growth  occurs  during  the  second  and  third  mouth  in  the  human  embryo, 
resulting  in  the  production  of  a  number  of  infoldings  of  the  surface,  which  are 
mostly  transverse  to  the  (bent)  axis  of  the  brain,  although  one  or  cwo  on  the  mesial 
surface  run  parallel  to  that  axis.  These  infoldings  of  the  surface,  which  may  be 
termed  temporary  ov primitive  sulci,  necessarily  have  a  corresponding  projection  into- 
the  cavity  of  the  thin-walled  hemisphere  vesicle  (fig.  75).  During  the  fourth  month, 
probably  owing  to  a  relatively  more  rapid  expansion  of  the  cranium,  most  of  these 
primitive  sulci  become  obliterated,  and  the  cerebral  surface  is  again  almost  smooth. 
Three,  however,  of  the  primitive  sulci  remain  as  permanent  fissures  of  the  brain,^ 
and  since  the  fissure  of  Sylvius  is  also  now  formed,  although  in  a  somewhat  different 
manner,  the  hemisphere  of  the  human  foetus  at  the  beginning  of  the  fifth  month  is 
marked  by  four  well  characterised  sulci  having  corresponding  projections  into  the 
interior  of  its  cavity.     These  permanent  primitive  sulci  are  the  following  : — 

1.  The  hippocampal  sulcus,  corresponding  with   the   projection  of  the  cornu 
ammonis  (hippocampus)  into  the  lateral  ventricle. 


^■nc.pt 


pew.  OCC. 


Fig.  82. — Foetal  brain  op  the  beginning  of  the  eighth  mojith.     (Milialkovics. ) 

A.,  from  above  ;  B.,  from  the  side  ;  C,  mesial  surface. 
i?o,  Rolandic  sulcus :  Sy,  Sylvian  fissure  ;  iMr.occ,  parieto-occipital ;  calc,  calearine  ;  p)'.c,  i^vecentral ; 
•pll,  parallel ;  int.par,  intraparietal ;  call. mar,  calloso-marginal  ;  unc,  iincus. 

2.  The  parieto-occipital  sulcus,  corresponding  with  the  bend  of  the  posterior 
cornu  of  that  ventricle. 

3.  The  calearine  sulcus,  corresponding  with  the  projection  of  the  calcar  avis. 

4.  The  Sylvian  fissure,  con*esponding  with  the  curve  of  the  lateral  ventricle. 

To  these  may  be  reckoned  the  longitudinal  infolding  of  the  mesial  wall  of  the 

^  It  is,  however,  uncertain  whether  the  temporary  sulci  develope  into  or  whether  they  are  replaced 
by  corresponding  permanent  sulci.  See  on  this  subject  a  paper  by  D.  J.  Cunningham  in  the  Journal  of 
Anatomy,  Aprjl,  1890. 


DEN  Kl.oI'MKNT    Ul'    THK    NKIIVES. 


T6 


(icmisphciv  just  l»eK>\v  the  liii»iKic-;iiiii»al  siiliiis,  wliicli  i>  (.•aiisL(l  liv  the  iujrrowth  of 

the  clioruid  plexus  of  the  lateral  veiitriele. 

AccordinK'  to  Eoker.  the  fiMsure  of  Sylvius  is  the  fii-st  of  the  primitive  sulci  to  ai)pear.  It  Ls 
vi.-ible  l)£'fort'  the  end  of  the  third  month,  as  a  wide,  shallow  depression,  which  divides  the  lower 
inarjrin  <'f  thf  hemisphere  into  two  nearly  e<iual  portions,  anil  at  the  Ijottom  of  which  is  seen 
the  thiekeninj:  of  the  Hour  of  the  vesic!  •  from  which  tlie  corjtu^  striatum  and  the  islanrl  of 
Keil  are  developed.  This  fissure  ajipears  to  \)v  fonuetl  by  a  curvin;r  of  the  still  thin-walleil 
hemisphere  vesicle  over  that  thickening,  around  which  the  vesicle  Ijends  ;  and  iU  anterior  ami 
j)osterior  parts  ultimately  meet  alony:  the  line  which  marks  the  posterior  liinb  of  the 
Sylvian  fissure  in  the  develojied  brain.  The  anterior  limb  is  profluced  much  Iat«r  by  a  further 
folding  over  of  that  jiart  of  the  mantle  which  i.s  in  front  of  the  fossa  Sylvii.  The  fissure 
remains  until  nearly  the  end  of  fietal  life  as  a  widely  open  depression,  at  the  bottom  of 
vhich  the  island  of  Keil  is  readily  visible.     It  closes  <,Tadually  from  behind  forwards. 

The  other  sulci  are  distiniruished  from  the  four  aliove-numerated  in  the  faet  that 
they  aiv  dejn-es.sious  of  the  surface  merely,  and  lutt  infoldinirs  of  the  whole  thickness 
of  the  wall  of  the  hemisphere  vesicle.^  They  begin  to  apjx-ar  about  the  end  of  the 
•fifth  month,  the  fissure  of  Rolando  being  the  first  to  show  (figs.  8<J  and  81). 

By  the  end  of  the  sixth  month  the  preceutml  and  inferior  frontal  sulci,  the 
intraparietal.  the  superior  occipital,  the  parallel,  the  inferior  temporal,  the  callo.so- 
marginal,  and  the  collateral  fissures  have  become  visilile,  as  well  as  the  anterior 
limb  of  the  Sylvian  fissure. 

Hy  the  end  of  the  seventh  month  (see  fig.  82)  most  of  the  remaining  principal 
convolutions  and  fissures  have  appeared,  and  those  which  were  previously  pre.sent 
have  increased  both  in  length  and  depth.  They  are,  however,  all  comparatively 
short  and  simple.  During  the  eighth  mouth,  they  (.ontinue  to  increa.se  in  length 
and  depth,  and  the  remaining  sulci  become  gradually  developed,  but  even  in  the 
ninth  month  there  are  none  of  the  accessory  or  secondary  fmTows  which  add  so  much 
to  the  complexity  of  the  developed  brain.  The  last  of  the  principal  sulci  to  make 
their  appearance  are  the  iuferica-  occipito-temporal. 

DEVELOPMENT    OF    THE    NERVES. 

Spinal  nerves. — At  an  early  period  of  development,  in  some  cases  even  before 
the  closure  of  the  neural  gi'oove,  in  others  during  or  shortly  after  that  event,  there 


Fig.   S3. — Traxsverse  section  throcgh  the  trcsk  of  ax    embryo 

SHARK,  TO  SHOW  THE  NEURAL  CREST.   (BalfoUT.) 

nc,  neural  canal  :  ]jr,  ganglion  rudiment  running  from  neural 
crest  ;  ar,  sub- notochordal  red  ;  ao,  aorta  :  sc,  parietal  mesoblast  :  sp, 
visceral  mesoblast  :  n^p,  muscle  plate  ;  mp',  portion  of  muscle  plate 
converted  into  muscle  ;  J'v,  portion  of  protoverteV)ra  wliicL  v,h\  give 
rise  to  the  vertebra  ;  al,  alimentaiy  canai. 


mp 


grows  out  bilaterally  from  the  angle  of  junction  of  the 
neural  with  the  general  epiblast  (fig.  89),  and  conse- 
quently at  the  dorsal  aspect  of  the  neural  tube  a  , 
continuous  ridge  or  crest  of  epiblast.  "which  was  first  de- 
scribed l)y  Balfour  in  elasmobranch  fishes  :  this  is  termed 
the  neural  crest.  At  intervals  along  the  sides  of  the 
neural  crest,  corresponding  with  the  middle  of  each 
mesoblastic  somite  or  protovertebra,  special  clavate 
enlargements  or  outgrowths  of  the  neural  crest  occur 

(fig.  83,  ^>;*).    These  grow  downwards  along  the  dorso-lateral  aspect  of  the  neural  canal, 
between  the  protovertebra  and  the  canal.     They  remain  foi-  a  time  attached  above 

'  An  exception  must,  however,  be  made  for  the  collateral  fi.«sure,  which  corresponds  with  the  col- 
lateral eminence  within  the  ventricle. 


74 


DEVELOPMENT    OF    THE    NEKVES. 


to  the  dorsal  aspect  of  the  neural  tube,  but  that  attachment  becomes  subsequently- 
lost,  and  they  then  form  completely  isolated  portions  of  epiblast,  composed  of  oval 
cells,  and  lying  at  the  side  of  the  embryonic  cord  between  it  and  the  muscle  plates 
of  the  protovertebrge  (fig.  84).  These  are  the  rudiments  of  the  posterior-root 
ganglia.  The  remainder  of  the  neural  crest  disappears  :  at  least  in  most  vertebrata. 
Some  little  time  after  the  sepai-ation  of  these  ganglion-rudiments,  the  ventral  or 
anterior  roots  of  the  spinal  nerves  begin  to  grow  out  from  the  ventro-lateral  aspect  of 
the  nearal  tube.  They  Avere  originally  described  by  Balfour  in  elasmobranchs,  as 
forming  bud-like  outgrowths  from  the  neural  epiblast,  the  outgrowths  being  com- 
posed of  spindle-shaped  cells  (fig.  84,  ar).     But  according  to  the  recent  and  extended 


Fig.  84. — Section  through  the  doksal  part  of  the  trunk  op  a  torpedo  embryo.     (Balfour.) 
pr,  rj,  n,  spinal  ganglion  rudiment ;  ar,  anteiior  root ;  c7i,  notochord  ;  nc,  neural  canal ;  mp,  muscle- 
plate. 

Fig.  85. — Section    of   the   ventro-lateral  angle    of   the  spinal  cord  of  a  pristiurus   embryo 
showing  the  outgrowth  of  an  anterior  root- rudiment.     (His.) 

«.?•,  axis  cylinder  processes  of  neuroblasts,  forming  the  anterior  root  ;  </,  germinal  cells  in  innennost 
part  of  wall  of  neural  canal. 

observations  of  His  in  various  classes  of  vertebrates,  what  actually  grow  out  to  form 
the  anterior  roots,  are  the  fibrous  prolongations  (axis-cylinder  processes)  of  neuro- 
blasts (v.  antea,  p.  58),  which  processes  converge  to  the  point  of  exit  of  the  root 
and  penetrate  gradually  into  the  adjoining  mesoblast  (fig.  85,  a.  r),  where  they 
come  into  close  contact  with  the  previously  formed  ganglion  rudiments  of  the  pos- 
terior roots.i 

The  fibres  of  the  posterior  roots  are  developed,  according  to  His,  as  processes 
from  the  oval  cells  of  the  ganglion  rudiment.  These  cells  are  in  fact  neuroblasts, 
and  from  either  end  of  each  cell,  which  is  thus  rendered  bipolar  (fig.  86),  a  process 
becoming  eventually  the  axis-cylinder  of  a  nerve-fibre  grows  out,  one  towards  the 
central  organ,  the  other  towards  the  periphery.  The  centrally  directed  processes 
soon  reach  and  grow  into  the  em.bryonic  cord  at  its  dorso-lateral  aspect,  where  they 
are  presently  seen  in  sections  occupying  an  oval  area  near  the  periphery  of  the  cord  ; 

1  The  place  at  which  the  anterior  roots  spring  from  the  cord  is  not  opposite  to  the  corresponding 
posterior  root,  but  midway  between  that  root  and  the  succeeding  one. 


("KANIAI-    NKKVES. 


75 


this  area  is  the  hoiiinning  of  thu  jfosteriur  white  cohimn  (fig.  K7) :  the  further  course 
and  attaehinents  of  tlie  ingmwinj^  fihres  in  the  cord  are  not  a<-curately  known,  but 
they  appear  to  bifurcate  and  extend  both  upwards  and  downwards  (Ilamon  y  (*ajal). 
Tht'  peripherally  directed  fibres  urow  downwai'ds  and  join  the  liundli;  of  fibres  of  the 
anterior  root,  to  foi-ni  tctoethi'r  with  them  the  mixed  s])injil  nerve  (fij^.  KS), 

The  axis-cyhnder  jn'ocesses  whicli  are  to  furni  the  fibres  of  the  anterior  I'oots 


Fig.  86. — Bipolar  cell  froji  si'inal  ganglion  of  a  4i  weeks  embryo.     (His.)     "fo 

Fig.  87. — Section  of  spinal  cori>  of  four  weeks  hcman  embrvc     (His.) 

The  posterior  roots  are  continued  within  the  cord  into  a  small  longitudinal  Lundle  which  is  the 
rudiment  of  the  posterior  white  column.  Tlie  anterior  roots  are  formed  by  the  convergence  of  the 
processes  of  the  neuroblasts.  The  latter,  along  with  the  elongated  cells  of  the  myelospongium  compose 
the  grey  matter.  The  exteraal  layer  of  the  cord  is  travei-sed  by  radiating  fibres  which  are  the  outer  ends 
of  the  spongioblasts.     The  anterior  commissure  is  beginning  to  appear. 

begin  to  make  their  appearance  about  the  beginning  of  the  fourth  week  in  the  human 
embiyo  (His).  Their  gi'owth  towards  the  periphery  is  slow ;  eTen  by  the  end 
of  the  second  month  they  hare  not  reached  the  tips  of  the  fingers  and  toes. 

Cranial  nerves. — The  neural  crest  is  continuous  along  the  dorsal  aspect  of  the 
cerebral  part  of  the  neural  titlje  as  far  as,  and  even  beyond  the  mid-brain.  As  in 
the  spinal  part,  clavate  enlargements  occur  here  also,  but  at  somewhat  ii-regular 
intervals,  and  form  ganglion  rudiments  which  become  separated  from  the  dorsal  aspect 
of  the  tube,  and  acquire  a  new  attachment  on  the  lateral  and  ventral  aspect.  Such 
ganglion  rudiments  have  been  described  for  the  third,  fifth,  seventh,  eighth,  ninth, 
and  tenth  nerves. 

In  the  chick  the  ganglion  rudiments  belonging  to  the  cranial  nerves  ajijiear  as  a 
thickening  of  the  cephalic  epiblast,  just  where  it  is  folding  round  into  the  as  yet 
unclosed  neural  canal  (fig.  89,  vg).  This  thickening,  according  to  Golowine,  is 
continuous  laterally  with  a  modified  portion  of  the  external  epiblast  (sensory 
epiblast),  and  soon  becomes  subdivided  into  three  ganglionic  groups,  and  these,  later, 
into  separate   ganglions.     There  is    a   corresponding  subdivision   of  the   sensory 


76 


CRANIAL    NERVES. 


epiblast  into  rudiments   of  special  sense  organs,  which  become  gradually  shifted 
downwards  and  outwards  away  from  the  ganglion  rudiments,  and  form  the  so-called 


Fig.  88. — Traksyerse  section  through  the  AMEPaoR  p.\rt  of  the  trunk  of  ak  embryo  of 

ScYLLiUM.      (Balfour.) 

sp.c,  spinal  cord  :  sp.g,  ganglion  of  ijo.sterior  root  ;  ar,  anterior  root;  dn,  dorsal;  sp.n,  Yentral 
branch  'of  spinal  nerve  ;  nvp,  muscle  plate  ;  wj/,  part  of  mnscle  plate  already  converted  into  muscle  ; 
miy.l,  part  of  muscle  plate  extending  into  the  limb  ;  inl,  nen'us  lateralis  ;  ao.  aorta  ;  cli,  notochord  ; 
s;-'.'/, 'sympathetic  ganglion  ;  ca.v,  cardinal  vein  ;  sd,  segmental  duct;  at,  segmental  tube  ;  du,  duode- 
num ;  Tip.d,  junction  of  hepatic  duct  with  it  ;  pan,  rudiment  of  pancieas  connected  with  another  part 
of  duodenum  ;  umc,  opening  of  ximbilical  canal  (vitelline  duct). 

"  branchial  sense-organs  "  of  Beard.     They  become  connected  subsequently  with 
outgrowths  from  the  posterior  nerve-rudiments. 

These  rudimentary  sense  organs  have  been  described  in  mammals  also  (by  Froriep).  They 
appear  to  represent  the  special  sense  organs  of  the  gill  clefts  of  fishes,  which  Avere  first  de- 
scribed by  Leydig  (18.50).  Beard  is  of  opinion  that  the  nose  and  ear  are  also  specialised 
branchial  sense  organs,  the  only  ones  that  are  persistent  in  higher  vertebrates.  This 
differentiation  of  a  special  sensory  portion  of  epiblast  into  rudiments  of  special  sense- 
organs,  occurs  according  to  Golowine's  observations  in  the  chick,  not  only  in  the  head  but  also 
in  the  trunk,  -^vhere  they  in  all  probability  represent  rudiments  of  "  organs  of  the  lateral  line," 
such  as  are  seen  in  fishes. 

Various  observers  have  described  the  ganglion  rudiments  as  actually  becoming  fonned   at 


OlIANIAL    NKltVES. 


77 


least  in  part  at  the  expense  of  the  •.eusory  fpil)la<tic  thickenin^rs.  In  connection  with  this 
question  it  is  worthy  of  note  that  from  the  sensory  thicki-ninjr  which  forms  the  olfactory  area 
a  }ranjrlionic  rudiment  becomes  formed  which  joins  the  olfactory  lobe  of  the  brain,  and  ;fivei> 
origin  to  the  olfactory  nerve-fibres  (His). 

Even  in  the  adult,  jus  \va.s  shown  by  Thorasen.  traces  of  pre-existent  ganglionic  structure 
can  be  found  in  the  root  of  the  third  nerve,  and  similar  traces  of  ganglionic  structure  have 


Fig.    89. — Tk.wsvekse     sectios 

THROIOH  THE  POSTERIOR 
PAKr  OF  THE  IIE.\I>  OF  AN 
EMBKYO    CHICK  <>F  :30    HOURS. 

(From  Ualfoiir.  'i 

hh,  hiiul-bniiii  ;  vj,  vagus 
nerve  ;  cp,  epiplast  ;  ch,  noto- 
clionl  ;  X,  sub-notochordal  rod  ; 
id,  throat ;  ht,  heart  ;  />/),  body- 
cavity  ;  so,  parietal  mesoblast  ; 
»f,  visceral  mesobla.st  ;  hy,  hypo- 
blast. 

also  been  described  by  Gaskell 
in  the  roots  of  the  fourth 
nerve,  in  the  motor  root  of  the 
fifth,  and  in  the  root  of  the 
seventh  nerve.  If  these,  as 
Oaskell  supposes,  indicate  the 
pre-existence  of  sensory  ele- 
ments in  the  roots,  it  is  pro- 
bable that  these  nerves  and 
ganglia  have  all  been  origi- 
nally developed  like  the  posterior  gangliated  roots  of  the  spinal  nerves,  as  ontgfrowths  from 
the  neural  crest.  "VMiether  they  aie  joined  by  outgi'owths  corresponding  to  the  efferent  fibres 
of  the  spinal  nerves,  or  whether  they  originally  contain  the  elements  of  the  efferent  fibres, 
and  thus  resemble  the  spinal  nerves  of  Amphioxus.  in  which  there  are  no  anterior  roots, 
but  both  sensory  and  motor  nerves  are  contained  in  the  posterior  roots,  is  not  at  present 
known.  A\Tiat  is  liowever  clear  is  that  the  ganglion  cells  and  afferent  fibres  in  the  roots  of 
the  third,  fourth,  motor  of  fifth  and  seventh  nerves,  eventually  entirely  disappear,  the  efferent 
fibres  alone  remaining,  while  in  the  roots  of  the  sixth,  eleventh,  and  twelfth  nerves  efferent 
fibres  only  are  found,  and  ganglionic  rudiments  are  not  develope<l  at  alL 

As  is  shown  in  another  part  of  this  work  (Neurology;  the  nuclei  of  origin  of  the  efferent 
cranial  nerves  are  disposed  in  two  longitudinal  series.  One  of  these  series  comprises  the  nuclei 
of  origfin  of  the  somatic  efferent  nerves  of  Gaskell.  which  conespond  with  the  largest  fibres 
of  a  typical  anterior  spinal  root,  and  the  series  of  nuclei  is  a  continuation  of  the  cell-column 
of  the  anterior  horn  ;  the  nuclei  of  this  series  are  those  of  the  byjwglossal,  or  twelfth,  the 
sixth,  fourth,  and  third.  The  other  series  comprises  the  nuclei  of  origin  of  the  splanchnic 
efferent  nerves  of  Gaskell.  which  corresjxjnd  vdth  the  medium-sized  and  smallest  fibres  of  a 
tj-pical  anterior  spinal  root,  and  the  series  is  a  continuation  mainly  of  the  cells  of  the  lateral 
■comu  or  intermediolateral  tract,  and  partly,  perhaps,  of  the  cells  of  the  base  of  the  posterior 
horn  ;  the  nuclei  of  this  series  are  those  of  the  spinal  accessory,  those  of  the  efferent  fibres 
of  the  vagus  and  glosso-pharyngeal.  the  facial,  and  the  motor  nucleus  of  the  fifth. 


According  to  the  observations  of  His,  the  distinction  into  an  anterior  or  ventral 
(somatic)  and  a  lateral  (splanchnic)  group  of  efferent  fibres  is  well  marked  in  the 
embrjo  by  the  fact  that  the  two  kinds  of  efferent  fibres  take  origin  from  entirely 
different  parts  of  the  basal  lamina  of  the  neural  tube,  those  which  correspond  with 
the  somatic  efferent  fibres  originating  from  groups  of  neuroljlasts  near  the  middle 
line,  while  the  others  take  their  rise  near  the  junction  of  the  basal  with  the  alar 
lamina  (see  fig.  90).  "With  the  alar  lamina  itself  the  afferent  fibres  are  con- 
nected, but  they  do  not  arise  from  groups  of  cells  within  it,  as  do  the  efferent  fibres 
Avithin  the  basal  lamina  :  on  the  contrary,  they  effect  their  connection  with  the  lamina 
by  growing  into  it  from  a  ganglion  in  the  manner  already  described  for  the  posterior 
roots  of  the  spinal  neiwes,  and  they  then  appear  in  most  cases  to  grow  downwards 
in  the  direction  of  the  spinal  cord.     This  is  stated  by  His  to  be  the  case  with  the 


78 


CRANIAL   NERVES. 


sensory  root  of  the  trigeminus,  which  thus  forms  the  white  bundle  known  as  the 
ascending  root  of  the  fifth,  but  which  in  its  actual  growth  is  descending  ;  and  also' 
with  the  afferent  fibres  of  the  glosso-pharyngeal  and  vagus  which  grow  downwards 
in  the  medulla  to  form  the  so-called  solitary  bundle.^  These  are  traceable  in  the 
adult  as  far  down  as  the  middle  and  lower  cervical  region  respectively,  bnt  in  the 
embryo  are  at  first  quite  short,  and  hmited  to  near  the  place  of  attachment  of  the 


Fig.  90,  A  and  B. — Sectioxs  across  the  hixd-braix  of  a  human  embryo,  10  mm.  long.     (His.)     *'. 

In  A,  the  origin  of  tlie  spinal  accessory  and  hypoglossal  nerves  is  shown,  the  fibres  of  both  arising 
from  gi'oups  of  neuroblasts  in  the  basal  lamina  of  the  neural  tube.  In  B,  one  of  the  roots  of  the  hyi^o- 
glossal  is  still  seen,  and  in  addition  the  root  of  the  vagus  nerve.  Tliis  is  represented  as  in  part  arising 
lilie  that  of  the  spinal  accessory  in  A,  from  a  group  of  neuroblasts  in  the  basal  lamina,  and  in  part  from 
a  bundle  of  longitudinally  coursing  fibres  placed  at  the  perijjhery  of  the  alar  lamina,  and  corresponding- 
in  situation  to  the  commencing  posterior  white  columns  shown  in  fig.  87. 


nerve  roots,  becoming  gradually  longer  as  development  proceeds.  The  solitary  bundle 
is  at  first  superficial,  like  the  ascending  root  of  the  fifth,  but  it  subsequently  becomes 
covered  in  by  the  bending  over  of  the  alar  plate,  and  the  formation  of  nervous  sub- 
stance in  this.  The  ganglion-rudiments  from  which  the  ingrowth  of  these  afferent 
fibres  takes  place,  become  the  Gasserian  ganglion  of  the  fifth,  and  the  jugular 
ganglia  of  the  glosso-pharyngeal  and  vagus. 

The  auditory  nerve-roots  appear  also  to  be  formed  by  an  ingrowth  from  the  cells 
of  its  ganglion-rudiment  into  the  alar  plate.  Subsequently  the  ganglion-rudiment 
becomes  subdivided  into  three  parts,  one  forming  an  intracranial  ganglion,  and  the 
others  giving  rise  to  the  branches  of  the  nerve  to  the  cochlea  and  vestibule  respectively 
(fig.  90,  C).  The  part  belonging  to  the  cochlea  (ganglion  cochleae)  forms  ultimately 
the  spiral  ganglion ;  while  the  one  on  the  vestibular  branch  forms  the  gangiiform 
swelling  of  Scarpa.  Prom  a  separated  part  of  the  ganglion  cochlese  the  nerve  to  the 
posterior  semicircular  canal  passes,  as  well  as  that  to  the  macula  of  the  saccule  ; 
from  the  vestibular  ganglion  the  nerves  to  the  other  ampulla3  and  to  the  utricle,  are 
derived.    The  geniculated  ganglion  of  the  facial  is  derived  from  the  same  ganglionic 

1  These  bundles,  therefore,  are  homologous  with  the  rudiments  of  the  posterior  white  columns 
which  are  derived  from  the  ingrowing  fibres  of  the  spinal  ganglia  (v,  antea,  p.  74). 


OLFACTORY    LoBK. 


7» 


mass  as  furnishes  the  ganu;lia  of  thu  audituiy  nerve,  but  its  cells  are  early  distin- 
guishable by  their  larger  size  and  clearer  ai)iiL'aranef.  From  these  cells  fibres,  which 
uve  probably  aticrent  (^His),  grow  centrally  into  the  hind-brain  and  peripherally  along 


m.G. 


WLC.v. 


WG.s. 


Fig.  90,  C. — Section  from  the  same  embryo  at  the  exit  of  the  facial  xerve,     (His.) 
(Several  sections  have  been  combined  to  form  this  figure. ) 

VI.,  fibres  of  sixth  nerve  taking  origin  fi-om  group  of  neuroblasts  in  basal  lamina  ;  XW.G.'j,  gang- 
lion geniculi  of  the  facial  ;  Ylll./r.i.c,  intracranial  ganglion  of  auditory  ;  VlU.Cr.r,  ganglion  vestibuli  j 
VIII.  6^.0,  ganglion  cochleae. 


the  nerve,  minghng  with  its  efferent  fibres.  Some  of  these  afferent  fibres  may  form 
the  chorda  tympani.  but  there  are  many  more  than  are  found  in  that  nerve. 

The  optic  nerves  take  origin  as  hollow  outgrowths  of  the  brain,  which  afterwards 
become  soHd,  while  nerve-fibres  become  developed  in  their  walls.  Their  mode  of 
origin  will  be  further  treated  of  in  connection  with  the  development  of  the  eye. 

The  olfactory  lobe,  which  consists  of  the  olfactory  bulb  and  tract  (often 
spoken  of  as  the  fii-st  or  olfactory  nerve),  and  the  part  of  the  base  of  the  brain 
from  which  the  tract  arises,  makes  its  appearance  as  a  protrusion  of  the  antero- 
ventral  part  of  each  cerebral  hemisphere,  extending  towards  the  thickened  olfactory, 
area  of  the  epiblast  (see  fig.  7u,  B,  Fd;  and  fig.  74,  olf).  This  primitive  olfactory 
lobe  is  early  seen  to  be  divided  into  anterior  and  posterior  parts  by  a  broad  sulcus 
(fig.  95).  Of  the  two  parts,  the  anterior  becomes  considerably  elongated,  and 
ultimately  forms  not  only  the  tract  and  bulb  but  also  the  trigonum  olfactoriimi, 
and  a  small  area  on  the  mesial  side  of  this  (termed  by  His  BroctCs  area)  ;  whilst 
from  the  posterior,  the  lai-ger  part  of  the  anterior  perforated  space  (mesial  to 
the  lateral  olfactory  root)    and    the    pedimcle   of    the   corpus   callosum   (gyms 


Fig.  91. — Ckaihial  nerves  of  a  HU^[A^•  embryo,  10.2  mm.  long.     (His.)     -J*. 

The  cranial  nerves  are  inflicated  by  Roman,  the  spinal  nerves  by  Arabic  numerals. 

€./(,  cerebral  hemisphere  ;  th,  thalamencephalon  ;  m.h,  mid-brain;  Mx,  maxillary  ijrocess  ;  Mn, 
mandibular  arch  ;  Hy,  hyoid  arch  ;  the  facial  nerve  is  seen  to  send  a  branch  (chorda  tympani)  aci'oss 
-the  hyoniandibular  cleft ;  (?.</,  Gasserian  ganglion  ;  eg,  ciliary  ganglion  ;  v,  vestibular,  and  c,  cochlear 
pai-t  of  auditory  ;  rj.p,  ganglion  petrosum  of  glosso-pharyngeal  ;  y.j,  ganglion  jugulare  of  vagus  ;  an 
anastomosis  is  seen  between  these  ;  g.  tr,  ganglion  trunci  of  vagus  ;  F,  ganglion  described  by  Froriep  as 
belonging  to  the  hypoglossal ;  r.d,  ramus  descendens  of  hypoglossal  •  ot  otic  vesicle.  The  eye  Ls  also 
represented,  and  a  part  of  the  heart. 


fig.  92. — Diagram  showing  the  centripetal  and  centrifugal  roots  of  the  cranial  nerves  of 

THE  same  embryo.  (His. ) 
The  places  of  exit  of  the  nerves  are  marked  by  dotted  circles  or  ovals.  The  efferent  nerves  (///., 
IV,  mV,  VI,  VII,  part  of  IX,  XI,  and  XII),  are  seen  to  arise  within  the  nerve  centre  from  groupsof 
neuroblasts  ;  the  afferent  fibres  (V.s,  VIII,  v  and  c,  most  of  IX,  and  X),  pass  a  cei-tain  distance  in- 
wards, and  for  the  most  part  also  caudalwards  in  the  nerve-centre,  and  there  end.  The  ganglion  rudi- 
ments from  which  they  have  grown  are  not  shown  here.     They  will  be  found  in  the  preceding  figure. 


SYMPATHETIC   NERVES.  81 

snbcallosus  of  Zuckerkandl)  are  developed.  These  parts  are  separated  even  in  tlie 
ailult  by  a  sulcus,  along  which  the  mesial  or  internal  olfactory  root  runs. 

The  olfactory  nerve  fibres  arise,  according  to  His,  from  neuroblasts  which 
become  formed  within  the  thickened  epiblast  of  the  olfactory  area  (see  p.  95). 
This  epiblast  at  a  certain  period  of  development  resembles  the  neural  epiblast,  and 
whilst  some  of  the  cells  become  spongioblasts,  others  become  pear-shaped,  or  spindle- 
shaped,  and  their  processes  grow  as  nerve  fibres  towards  the  olfactory  lobe.  Not 
only,  however,  do  these  fibres  emerge  from  the  olfactory  epiblast,  but  some  of  the 
neuroblasts  themselves  also  pass  out,  and  these  form  a  ganglion  which  lies  between 
the  olfactory  lobe  and  the  olfactory  area.  Subsequently  this  ganglion,  the  cells 
of  which  are  prolonged  at  either  end  into  nerve  fibre  processes,  becomes  attached 
to  and  partially  invests  the  olfactoiy  bulb,  with  which  it  ultimately  blends,  forming 
the  part  whence  the  olfactory  nerve  fibres  pass  to  the  Schneiderian  membrane 
(layer  of  olfactory-glomeruli  and  nerve  fibres),  whilst  bands  of  fibres  on  the  other 
hand  grow  centripetally  and  become  the  olfactory  roots.  These  are  in  fact 
comparable  to  the  centripetal  (so-called  "  ascending ")  roots  of  the  trigeminus, 
glosso-pharyngeal,  and  vagns. 

It  is  not  until  the  third  month  that  the  part  of  the  olfactory  lobe  which  forms 
the  bulb,  begins  to  grow  forwards  away  from  the  trigonum,  and  thus  to  form  the 
olfactory  tract. 

The  cranial  nerves,  except  the  optic  and  olfactory,  and  the  relations  they  bear  to  one 
another  and  to  the  visceral  arches  of  the  head,  are  shown  in  &g.  91  as  they  occur  in  the  human 
embryo  of  about  four  weeks.  Fig.  92  distinguishes  diagrammatically  the  nerves  whi'jh  grow 
into  the  nerve  centres  (centripetal  or  afferent  nerves)  from  those  which  grow  out  from  the 
centres  (centrifugal  or  efferent  nerves),  and  the  extent  of  growth  inwards  of  the  former  in  the 
€ame  embryo. 

The  S3rinpatlietic  nerves  and  ganglia. — That  these  are  merely  outgrowths 
of  the  cerebrospinal  nervous  system,  nearly  all  recent  observations,  both  morpho- 
logical and  physiological,  clearly  show.  But  even  before  this  fact  had  come  to  be 
generally  recognized,  it  was  known  that  they  are  developed  in  connection  with  the 
«pinal  nerves  (Balfour),  and  indeed  as  offshoots  from  the  posterior  spinal  ganglia 
(Schenk  and  Birdsell,  Onodi).  They  appear  for  a  time  as  enlargements  upon  the 
main  stem  of  each  spinal  nerve,  but  afterwards  become  connected  A\'ith  this  by  a 
short  branch  (r.  communicans)  (fig.  88),  and  with  one  another  by  a  longitudinal 
commissure.  The  branch  in  question  contains  the  splanchnic  fibres  of  the  spinal 
nerve,  and  the  sympathetic  ganglia  are  its  splanchnic  or  vagrant  ganglia  (Gaskell). 
The  splanchnic  ganglia  of  the  cranial  nerves  are  probably  formed  in  a  similar  way, 
but  their  mode  of  development  has  not  as  yet  been  worked  out. 

The  ciliary  ganglion  appears  to  be  formed  as  an  outgrowth  of  the  Gasserian  ganglion  (fig. 
91,  e.g.)  much  in  the  same  way  as  the  sympathetic  trunk  ganglia  are  formed  as  offshoots  of  the 
posterior  spinal  ganglia.  In  elasmobranchs  it  is  derived  from  the  ophthalmicus  profundus 
ganglion,  itself  an  offshoot  of  the  Gasserian  (Ewart). 

Paterson  has  recently  described  the  sympathetic  chain  of  ganglia  as  developing  in  mammals 
(rodents)  from  a  continuous  rod  of  mesoblast  lying  on  either  side  of  the  aorta,  and  as  becoming 
only  secondarily  segmented  and  connected  %vith  the  cerebro-spinal  nei-ves.  But  observations 
upon  earlier  embiyos  than  were  used  by  Paterson  are  necessary  before  the  mesoblastic  origin 
of  the  rod  can  be  admitted. 

RECENT    LITERATURE. 

Ahlbom,  Uiler  die  Bedeutung  der  Zirhddriise.     Zeitschr.  f.  -wissensch.  Zoologie.     Bd.  xl.,  1884. 

Barnes,  Will.,  On  the  development  of  the  posterior  fissure  of  the  spinal  cord  and  the  reduction  of 
the  central  canal  in  the  pig.     Proc.  Americ.  Acad.  Arts  and  Sciences,  1884. 

Beard,  J.,  On  the  cranial  ganglia  and  segmental  sense  organs  of  fishes.  Zoolog.  Anzeiger,  1885  ; 
The  system  of  branchial  sense  organs  and  their  associated  ganglia  in  Ichthyopsida.  Quarterly  Joximal 
of  Micr.  Science,  1885  ;  The  development  of  the  peripheral  nerxom  system  of  vertebrates.  Quarterly 
Journal  of  Micr.  Science,  Oct.,  1888. 

Bedot,  III.,  Recherches  sur  le  ddveloppement  des  nerfs  spinaux  chez  les  Tritons.  Recueil  zoolog. 
Suisse,  i,,  2,  1884. 


82  RECENT   LITERATURE. 

Beer,  B.,  On  the  development  of  the  Sylvian  fissure  in  the  human  emhryo.  Journal  of  Anatomy 
BBd  Physiology,  1890. 

Beraneck,  E.j  Recherches  sur  le  developpement  des  nerfs  crdniens  chez  les  lizards.  Recueil" 
zoolog.  Suisse,  1884  ;  ^tude  sur  les  replis  midullaires  du  poidet.     Recueil  zoolog.  Suisse,  iv.,  1887. 

Chiarugi,  Sidlo  sviluppo  di  alcuni  nervl  cerehrali  e  spinali.     Anat.  Anzeiger,  1889. 

Cunningham,  D.  J.,  The  complete  fissures  of  the  human  cerebrum,  and  their  sif/nificance  in^ 
connection  with  the  growth  of  the  hemisphere  and  the  appearance  of  the  occipital  lobe.  Journal  of 
Anatomy,  April,  1890. 

Dohrn,  A.,  Die  Entstehung  dei'  Hypophysis  bei  Petromyzon  Planeri.  Mittheil.  aus  der  zoolog. 
Station  zu  Neapel,  iv. ,  i. ,  1883;  Ueber  die  erste  Anlage  und  Entwicklung  der  motor ischen  Kuchen- 
marksnerven  bei  den  Selachiern,  Mittheil.  aus  der  zoolog.  Station  zu  Neapel,  Bd.  viii.,  1888. 

E-wart,  J.  C,  On  the  development  of  the  ciliary  or -motor  oculi  ganglion.  Proc.  Roy.  Soc.  xlvii., 
1890. 

Froriep,  A.,  Ueber  ein  Ganglion  des  Hypoglossus  u.  Wirbelanlagen  in  der  Occipitalregion, 
Arch.  f.  Anat.  u.  Physiol.,  Anat.  Abth.j  1882  ;  Ueber  Anlagen  von  Sinnesorganen  am  Facialis,  Glosso- 
pharyngeus  und  Vagus  und  iiber  die  genetische  Stellung  des  Vagus  zum  Hyjvjglossus  und  iiber  die 
Iferkunft  der  Zungenmusculatnr,  Arch.  f.  Anat.  u.  Physiolog.,  Anat.  Abthl.,  1885. 

G-oette,  A.,  Ueber  die  Entstehung  und  die  Homologien  des  Hirnanhangs,  Zool.  Anzeiger,  1883. 

Grolo-wine,  E.,  Sur  le  developpement  du  systeme  ganglionnaire  chez  le  poulet,  Anat.  Anzeiger,  1890 

Graaf,  H.  W.  de,  Bijdrage  tot  de  kennis  van  den  bouw  en  de  ontwilckeling  der  epiphyse  bij 
Amphibien  en  Eeptilien.     Proefschrift,  Leiden,  1886. 

His,  W.,  Ueher  das  Auftreten  der  weissen  Substanz  und  der  Wurzelfasern,  am  RiicTcermiarlc- 
mensehlicher  Embryonen,  Archiv  f.  Anat.  u.  Phys.,  Anat.  Abth.,  1883;  Zur  Geschichte  des  mensch- 
lichen  Rilckenmarks  und  der  Nervenwurzeln,  Abhandl.  d.  math. -phys.  Kl.  d.  kgl.  Sachs.  Gesellsch.  d. 
Wissensch.,  Bd.  xiii.,  No.  6,  1886  ;  Die  Entwicklung  der  ersten  Nervenbahnen  beim  menschlichen 
Embryo.  Uebersichtliche  Darstellung,  Archiv  f.  Anat.  u.  Physiol.,  Anat.  Abth.,  1888  ;  Zur  Geschichte- 
des  Gehirns  soivie  der  ventralen  und  peripherischen  Nervenbahnen  beim  menschlichen  Embryo,  Abhandl. 
d.  math. -phys.  Kl.  d.  kgl.  Siichs.  Gesellsch.  der  Wissensch.,  Bd.  xiv.,  1888  ;  Die  Formentivicklung  des: 
menschlichen  Vorderhirns,  Abhandl.  d.  konigl.  Sachsischen  Gesellschaft,  Bd.  xv.,  1889  ;  Die  Neurohlnsten 
u.  dfren  Entstehung  im  embryonalen  Mark,  Abhandl.  d.  konigl.  Sachs.  Gesellschaft,  Bd.  xv.,  1889. 

His,  "W.,  jun.,  Zur Entwicklungsgesch,  d.  Acustico-facialis-gebietes  beim  Menschen,  Arch.  f.  Anat. 
ti.  Physiol.,  Anat.  Abth.  1889. 

Hoffmann,  C.  K. ,  Ueber  die  Metamerie  des  Nachhirns  u.  Hinterhirns  u.  ihre  Beziehung  z.  d. 
segmevtalen  Kopfnerven  bei  Reptilienembryonen,  Zool.  Anzeiger,  xii. 

Johnson,  A.,  and  Sheldon,  Lilian,  On  the  development  of  the  cranial  nerves  of  the  neivt,  Pro- 
ceed, of  the  Royal  Society,  vol.  xl.,  1887. 

Kaczander,  J.,  Ueber  die  Beziehmgen  des  Medullarrohrs  zu  dem  Primitivstreifen,  Wiener- 
Medicin.  .Tahrb.,  1886. 

Kraushaar,  E.,  Entwicklung  der  Hypophysis  und  Epiphysis  bei  Nagethieren,  Zeitschi-.  f. 
wissensch.  Zoologie,  1884. 

Kupffer,  Primdre  Metamerie  der  Neuralrohrs  der  Vertehraten,  Miinchen.  Sitzungsb.,  Bd.  xv. 

McClure,  The  primitive  segmentation  of  the  vertebrate  brain,  Zool.  Anzeiger,  xii. 

Marshall,  A.  Milnes,  On  the  early  stages  of  development  of  the  nerves  in  birds,  Journal  of  Anatomy 
and  Physiology,  1877  ;  The  development  of  the  cranial  nerves  in  the  chick.  Quarterly  Journal  of  Microsc. 
Science,  1878  ;  On  the  head  cavities  and  associated  nerves  of  elasmobranchs.  Quarterly  Journal  of- 
MioTosc.  Science,  1881. 

Mihalkovics,  v.,  Wirbelsaite  u.  Himanhang,  Archiv  f.  mikr.  Anatomic,  1875  ;  Entwicklung s- 
geschichte  des  Gehirns,  Leipzig,  1877. 

Onodi,  A.  D.,  Ueber  die  Entivicklung  der  Spinalganglien  und  der  Nervenwurzeln,  Internat. 
Monatsschr.  f.  Anat.  u.  Histologic,  i.,  3,  1884  ;  Ueber  die  Entwicklung  des  sympathischen  Nerven- 
sy stems,  Arch.  f.  mikr.  Anat.,  Bd.  xxvi.,  1885. 

Orr,  H.,  Note  on  the  development  of  Amphibians,  chiefly  concerning  the  central  nervous  system,  d-c. 
Quarterly  Journal  of  Micr.  Science,  xxix.,  1889. 

Osbom,  The  origin  of  the  corpus  callosum,  Morph.  Jahrbuch,  1887. 

Paterson,  A.  M.,  On  the  fate  of  the  muscle-plate,  and  the  development  of  the  spinal  nerves  and' 
limb  pilexuses  in  birds  and  mammals.  Quarterly  Journal  of  Micr.  Science,  Aug.,  1887  ;  The  development 
of  the  sympathetic  nervous  system  in  mammals.     Proc.  Roy.  Soc,  April,  1890. 

Rabl,  C,  Bemerkung  iiber  die  Segmentirung  des  Hirns,  Zoolog.  Anz. ,  1885. 

Rabl-Ruckard,  Gehirn  der  Knochenfische,  Arch.  f.  Anat.  u.  Physiol.,  Anat.  Abth.  1882  und 
1883. 

Robinson,  A.,  On  the  development  of  the  posterior  columns,  of  the  posterior  fissure,  and  of  the 
central  canal  of  the  spinal  cord.     Owens'  College  Studies,  1890. 

Rudingrer,  Ueber  die  BUdung  der  Augenblasen,  Sitzungsb.  der  Gesellsch.  f.  Moi-phol.  zu  Miinchen, 
1889. 

Spencer,  B.,  On  the  presence  and  structure  of  the  pineal  eye  in  LacertUia,  Quarterly  Journal  of 
Micr.  Science,  1886. 

Strahl,  H.,  und  Martin,  E.,  Die  Entwicklung  des  Parietalauges  bei  Anguis  fragUis  und  Zacerta- 
vivipara,  Arch.  f.  Anat.  u.  Phys.,  Anat.  Abth.  1888. 

Vigrnal,  W.,  Sur  le  developpement  des  eliments  de  la  moelle  des  mammifhres,  Archives  de  physiol. 
1884  ;  Recherches  sur  le  developpement  de  la,  substance  corticale  du  cerveau  et  du  cervelet.  Archives  de 
physiologie,  1888. 

"Wijhe,  J.  "W.  van,  Ueber  Somiten  und  Nerven  im  Kopfe  von  Vogel-  und  Reptilienembryonen, 
Zoolog.  Anzeiger,  1886, 

Zuckerkandl,  E.,  Ueber  das  Riechcentrum,  Stuttgart,  1887. 


DEVELOPMENT    OF    THE    EYE. 


8a 


DEVELOPMENT    OF    THE    EYE. 

The  first  devektpiucnt  of  the  eye  occurs  us  a  lioUow  protrusion  of  the  antj)dor 
cerebraJ..vesicle  — primaiy.QpiiiJ.^'esicle— in  the  manner  ah-eady  mentioned  (see 
fio-.  08,  Pj'and  fij;.  0:5).  The  vesicle  thus  formed  abuts  externnliy  against  the 
e^enUjJUpihliist  of  the  side  of  the  head  (fi,u-.  OG)  ;  and  this  external  epibl.ast  opposite 
the  most  proniineiit  point  of  the  priuuay  oi)tLc_vesk;le,  becomes  tiiicla-ned  and   in- 

O.k. 


oi/: 


Fig.  94. — SlUF.  VIEW  OF  ANTF.RIOR 
PART  (IF  DilAIN  OF  SIORK  A1>VAXCKD 
IIU.MAX  KMBKYO,  SHOWING  THB 
I'lilMAllY  Ul'TIC  VESICLK  FOLDED 
AND  CUI'PKI).       (His.) 

c./(,  cereliml  lieniisi^liere (part  of); 
olj.,  uifactory  lobe  ;  opt.,  optic  cup. 


Fig.    93,   A, — Braik   op   chick   op   2nd   day, 

VIEWED  FROM  BELOW,  TO  SHOW  THE  FORMA- 
TION OF  THE  OPTIC  VESICLES  BY  OUTGROWTH 
OF  THE  SIDE  OF  THE  FORE-BRAIN,  AND  AT 
THE  SAME  TIME  BY  THE  FOLDING  OVER  OF 
THE  ENLARGED  PART,  THE  PRODUCTION  OP 
A    GROOVING    OR    CUPPING    OF    THE    VESICLES. 

(His.) 

f.h'.,   m.br.,   h.br.,  fore-,   mid-,    and  hind- 
brain  ;  opt.,  optic  vesicle  ;  i,  infundibulum. 

Fig.    03,  B. —  Brain    of    human    embryo    op 

THREE  WEEKS,  SHOWING  THE  PRIMARY 
OPTIC  VESICLES  AS  OUT-GROWTHS  FROJI  THE 
FOREBRAIN.       (His.) 


Fig.  95. — Side  view  op  the  same  part  op 

THE  brain    IN      A      STILL     MORE    ADVANCED 
EMBRYO,   THE  EYE  HAVING  BEEN  CUT  AWAY. 

(His.) 

opt.,  cut  end  of  optic  stalk,  .showing  the 
manner  in  which  it  is  folded  ;  i,  infundibu- 
lum ;  olf.p. ,  posterior  part  of  olfactory  lobe  ; 
olf.a.,  anterior  part  of  the  same  ;  c.h.,  cere- 
bral hemisphere  ;  t.c,  tubes  cinereum. 


vag'inated,  so  as  to  fonn  at  first  a  holloF  cup-shaped  depression  with  thickened  walls 
(figs.  97,  98),  and  subsequently  by  the  closing  in  of  the  epiblast  at  the  mouth  of  the 
cup,  a  hollow  island  of  epithelial  cells  (fig.  99).  This  island,  which  is  the  mdiiae-ntary 
lens,  lies  l^etween,  but  is  entirely  distinct  from  the  external  epiblast  on  the  one  han"d, 
aha'  the  neural  epiblast  of  the  primary,  optic  vesicle  on  the  other  hand.  Its  forma-- 
tion  is  accompanied  by  a  cupping  in  of  the  primary  optic  vesicW figs.  94,  97,  98), 
which  is  invaginated  before~lf7'ahd  this  invagination  is  increased  by  an  ingrowth 
of  Eoesoblast,  which  occurs  between  the  lens  and  the  cupped  opUc  _y^icle,  and 
which  sulDsequently  forms  the  vitreous^humour,  Invaginated  in  this  way  the 
cavity  of  the  original  optic  vesicle  becomes  almost  entirely  oljliterated,  and  appears 
merely  as  a  cleft  between  the  two  layers  which  form  the  wall  of  the  so-called  "  optic 
cup."  The  inner  of  these  two  layers  is  from  the  first  thicker  than  the  outer,  and  in 
it  are  developed  all  the  parts  of  the  future  retina  from  the  membrana  limitans  interna 

G  2 


84 


DEVELOPMENT    OF    THE    EYE. 


to  the  layer  of  rods  and  cones,  Avhile  from  the  outer  thinner  layer  the  hexagonal 
pigmented  epithelium  of  the  retina,  with  its  continuation  into  the  uvea,  is  foimed. 
The  invagination  of  the  primary  optic  vesicle  does  not  occur  only  opposite  the 


Fig.  96. — Part    of  a    section    throcgh  the 

HEAD  OF  AN  EARLY  HUMAN  EMBRYO,  SHOWING 
THE  CONNECTION  OF  THE  PRIIFARY  OPTIC  A'ESI- 
OLES   "SVITH    THE    FOREBRAIN.        (His.) 

olf,  olfactory  area  of  epiblast  ;  c.A,  part  of 
forebrain  which  gives  rise  to  cerebral  hemi- 
spheres ;  ih,  thalamencephalon  ;  p.o.v,  primaij 
optic  vesicles. 


Fig.  97. — Section  through  the  same  part  op  a 

MORE  ADVANCED  EMBRYO,  IN  WHICH  THE  LENS  IN- 
VAGINATION IS  FORMED,  AND  THE  PRIMARY  OPTIC 
VESICLES    ARE    CUPPED.       (His. ) 

o.c,  optic  cup  ;  o.s,  optic  stalk  ;  I,  lens  invagina- 
tion ;  c.h,  cerebral  hemispheres  ;  th,  thalamencepha- 
lon ;  i,  infundibulum  ;  olf,  olfactory  area. 


Tn^. 


0- 


C.& 


Fig.  98. — Vertical  section  thkough  the  jiiddle  of  the  developing  eye  of  a  chick  of  the 

THIRD    DAY.        (E.    A.    S. ) 

The  section  passes  longitudinally  through  the  deficiency  in  the  lower  part  of  the  optic  cup,  and 
shows  the  mesoblast  extending  in  between  the  lens  invagination  and  the  pigment  layer  of  the  optic  cup. 

</taZ,  thalamencephalon ;  to.  cp,  neural  epiblast;  c.qj.  cutaneous  epiblast;  o.s.,  optic  stalk  ;  o,  o, 
cavity  of  primary  optic  vesicle  ;  me,  mesoblast ;  v,  mesoblast  passing  behind  lens  to  form  vitreous ; 
I,  lens  invagination. 

Fig.  99. — Section  through  the  eye  and  optic  stalk  of  a  human  embryo  of  five  weeks.    (His.) 

W.C.,  connection  of  optic  stalk  with  thalamencephalon  ;  .Sj3,  cleft  or  fold  in  the  stalk,  where  the 
drteria  centralis  retinse  passes  in  ;  P,  pigment  layer  ;  R,  retina  ;  L,  lens. 


DEVELOPMEXT    OF   THE    EYE.  85 

place  where  the  lens  is  becoming  involuted,  but  also  below,  or  ventral,  to  that  place, 
so  that  a  section  exactly  throu_<;-li  the  middle  of  thi-  ojitic  cup  at  rijrlit  au<i:les  to  the 
axis  of  this  part  of  tlif  head,  shows  a  gap  iu  the  boundary  of  the  cup  through  wliich 
the  mesoblast  is  passing  into  the  space  between  the  lens  and  the  invaginated  optic 
vesicle  (fig.  \)S,  r).  This  gap  or  cleft  soon  becomes  closed,  but  the  suture  or  line  of 
closure  long  remains  apinu'cnt  from  the  fact  that  when  pigment  Iiegiiis  to  be  de- 
posited in  the  eye,  tiiis  so-called  dwroidal^ssure  remains  for  soinfi  tiine  uiipigmented 
(until  the  sixtli  week  in  man).  "  ~~ 

The  ventral  invadiuition  is  in  mammals  continued  for  a  c(»nsiderable  distance 


into  the  stalk  of  the  optic_  vesicle  (fig.  95),  and  the  simultaneous  inclosure  of 
mesoblastic  tissue  leads  to  the  introduction  of  the  central  bl()od=TPsstriB  of  the  retina 
withni"tEeo£tigjp.vye.     In  birds  no  such  infolding  of  the  stalk  occurs. 

The  lower  invagination  of  the  optic  cup  serves  not  only  to  peiTnit  of  the  passage  of  meso- 
blast behind  the  lens  for  the  formation  of  vitreous  humour,  but  also  to  establish  a  direct 
connection  between  the  nerve-fibres  which  are  formed  along  the  course  of  the  optic  stalk 
(future  optic  nerve)  and  the  centre  of  the  inner  laj-er  of  the  optic  cup  (future  retina)  (0. 
Hertwig). 

The  malformation  termed  colohoma  iridin  is  attributed  to  a  persistence  of  the  choroidal 
cleft,  which  extends  behind  the  iris  along  with  the  retinal  pigment  or  uvea,  as  far  as  the 
margin  of  the  pupil. 


Fig.  100. — Horizontal  section  throcgh  the  eye 

OF  AN  EMBRYO    RABBIT   OF    TWELVE  DAYS  AND  SIX 

HOURS.     '-°.     (Kolliker. ) 

o,  optic  stalk ;  h',  remains  of  the  ca^-ity  of  the 
primaiy  optic  vesicle  ;  p,  proximal  lamella  of  the 
optic  cup  (pigmentum  nigrum)  ;  ?•,  distill  lamella 
(retina)  ;  I,  lens  invagination,  ^^•idely  open  at  ol  ; 
V,  papillar  elevation  in  the  bottom  of  the  lens 
vesicle  ;  m,  mesoblast  ;  y,  mesoblast  of  vitreous  ; 
r,  a  blood-vessel  at  the  anterior  border  of  the  optic 
cup  ;  e,  cutaneous  epiblast. 


Fig.  101.— Eyeball  of  a  human  embryo  op 

FOUR     weeks    cut    ACROSS,     AND     THE    ANTERIOR 
HALF  REPRi-SENTED  FROM  BEHIND.  (KoUiker.  )  ^2°, 

fv,  the  remains  of  the  cavity  of  the  primary 
optic  vesicle  ;  p,  outer  layer  forming  the  retinal 
pigment  ;  r,  the  thickened  inner  part  giving  rise 
to  the  columnar  and  other  structures  of  the  retina  ; 
V,  commencing  vitreous  humour  within  the  optic 
cup  ;  v',  the  cleft  throuj;h  which  a  vascular  loop, 
a,  projects  from  below  ;  /,  the  lens  with  a  central 
cavity. 


The  hollow  optic  stalks  are  at  first  freely  in  communication  with  the  thaiam- 
encephalon.  or  third  ventricle.  Xerve-fibres  grow  along  their  walls,  from  neuroblasts 
which  develop  iu  the  retinal  epiblast,  and  pass  towards  the  nerve-centre  (His),  and 
the  cavities  of  the  stalks  become  thereby  gradually  obliterated,  the  radially  striated 
epithelial-like  arrangement  of  the  wall  being,  however,  long  evident.  A  new  con- 
nection becomes  subsequently  established  between  the  posterior  part  of  the  optic 
stalks  (optic  tracts)  and  the  mesencephalon,  whilst  the  middle  parts  become  united 
with  one  another  to  form  the  chiasma. 


86 


DEVELOPMENT    OF    THE    EYE. 


The  development  of  the  retina  from  the  inner  layer  of  the  optic  cup,  has  not 
been  fullyl\^orked  out.  In  its  earlier  stages  it  closely  resembles  in  structure  the 
wall  of  the  cerebral  vesicles,  consisting  of  elongated  ..epithelium-like  cells,  apparently 
arranged  in  several  interlocking  layers.  Of  these  celjs  some  become  developed  into 
nerve-fibres  and  nerve-cells  (inner  granules  and  ganglionic  layer),  others  into  susten- 
tacular  tissue,  similar  to  the  neuroglia  of  the  central  nervous  system  (molecular 
layers,  MiiUerian  fibres),  whilst  the  outermost  layer  forms  the  sense- epithelium  (W. 
Miiller),  or  layer  of  outer  granules,  which  is  sharply  marked  oflF  against  the 
layer  of  hexagonal  pigment  cells  by  the  membrana  limitans  externa,  as  is  the  nerve- 
fibre  layer  from  the  vitreous  humour  by  the  membrana  limitans  interna.  For  a  long 
time  there  is  no  trace  of  the  rods  and  cones.  These  begin  to  appear  some  little  time 
before  birth  in  man  and  most  animals,  but  in  animals  which  are  born  blind,  such  as 
kittens,  not  until  after  birth  (M.  Schultze),  in  the  shape  of  small  protuberances  of 
the  sense-epithelium  cells  growing  beyond  the  limitans  externa,  and  forming  at  first 
the  inner  segments  of  the  rods  and  cones,  and  subsequently  the  outer  segments  also. 
The  latter  as  they  are  developed  become  imbedded  in  the  inner  surface  of  the 
bexagonal  pigment  cells,  which  have  become  developed  from  the  outer  layer  of  the 
optic  cup. 

The   anterior   third  of  the  optic  cup  does  not   undergo  the   changes   above 


af.n-- 


,.''!^ 


6.6 


Fig.  102. — Section  through  the  eye  of  a  babbit  embryo,  mobe  advanced  in  development  thait 

THAT    SHOWN    IN    FIG.    98.       (BalfoUP.) 

c,  e-pithelium  of  cornea;  I,  lens;  me.c,  mesoblast  growing  in  to  form  the  substantia  propria  of  the 
cornea  ;  o/'. «,  optic  ner\'e  ;  rt,  retina  ;  a.c.r,  mesoblast  for  the  formation  of  the  vitreous  humour,  and 
the  arteria  centralis  retinffi. 

described.  ■  Its  two  layers  become  here  developed  into  the  comparatively  simple  pars 
ciliaris  retinse,  and  in  front  of  the  ciliary  region  they  extend  forwards  and  inwards 
in  front  of  the  lens  and  in  close  contact  with  the  back  of  the  iris,  where  they  form 
the  thickly  pigmented  epitheliiun,  which  is  known  as  the  uvea  and  terminates  at  the 
margin  of  the  pupil. 

rurther   development   of    the   lens. — The   hollos: epiblastic    vgsicle  from 

which  the  lens  developes  is  composed  of  a  thick  posterior  and  a  thin  anterior  layer 
which  pass  into  one  another  at  the  equator  of  the  lens,  and  enclose  a  clear  fluid. 
In  mammals,  the  vesicle  when  first  formed  also  contains  a  small  mass  of  epithelium 
cells  which  have  become  separated  ofP  from  the  posterior  wall  (fig.  100),  but  these 


THK    VITREOUS    HUMOUli.  87 

afterwards  disappear.  The  thin  anterior  layer  remains  throughout  life  as  a  simple 
layer  of  cubical  cells,  and  forms  the  so-called  lens-epithelium ;  but  the  cells  of  the 
posterior  layer  grow  forwards  into  the  cavity  of  the  lens-vesicle  as  the  lens-fibres  : 
those  in  the  middle  being  the  longest  and  straiglit,  while  the  rest  are  slightly  curved 
with  their  concavity  towards  the  equator,  and  become  gradually  shorter  towards  the 
circumference,  where  they  pass  through  gradually  shortening  columnar  cells  {transi- 
tional zone)  into  continuity  with  the  anterior  epithelium.  By  the  growth  of  these 
fibres  the  cavity  of  the  lens-vesicle  becomes  obliterated. 

In  this  manner  the  central  part  of  the  lens  is  formed,  and  it  consists  in  the  main 
of  fibres  whicii  pass  in  an  antero-posterior  direction.  The  remainder  of  the  lens  is 
Ibrnifd  of  fibres  which  are  so  disposed  as  to  curve  round  its  margin  and  over  the  ends  of 
the  first  formed  fibres  ;  they  are,  moreover,  deposited  in  successive  layers  and  in 
three  (or  more)  separate  sections,  so  that  their  ends  abut  against  one  another  in  front 
and  behind  along  tri-radiate  (or  multi-radiate)  lines,  such  as  may  be  seen  in  the 
macerated  lens.  These  later  deposited  fibres  are  all  formed  at  the  equator  (at  the 
transitional  zone),  where  chiefly  cell-multiplication  takes  place,  and  they  grow  hence 
meridionally  backwards  over  the  ends  of  the  already  developed  antero-posteriorly  dis- 
posed fibres  of  the  central  part  of  the  lens. 

The  capsTile  of  the  lens  is  early  visible  as  a  thin  homogeneous. membrane,  the 
origin  of  which  is  still  undetermined.  According  to  some  observers  (Lieberkiihn, 
Arnold,  Lowe)  it  is  derived  from  a  thin^layer  of  mesoblast,  which  passes  in  between 
the  lens  and  the  optic  cup  ;  according  to  others  (K6lliker,T^essler,  Balfour),  it 
appeal's  before  any  mesoblast  has  passed  in,  and  they  therefore  regard  it  as  a 
cuticular  deposit  fi'om  the  lens  cells.  In  the  human  embryo,  His  figures  mesoblast 
as  existing  from  the  first  between  the  lens  invagination  and  the  optic  cup  (v.  fig.  97). 

In  connection  vnth.  this  question  it  must  be  remembered  that  the  substantia  propria  of  the 
•cornea  (see  below),  which  is  formed  of  connective  tissue,  and  is  therefore  mesoblastic  in 
nature,  also  at  first  makes  its  appearance  as  a  homogeneous  deposit  before  any  mesoblast  cella 
have  passed  in  behind  the  corneal  epithelium.^  Its  chemical  natuie,  and  its  continuity  at 
the  equator  with  the  suspensory  ligament  and  hyaloid  membrane,  certainly  point  to  the  lena 
capsule  as  being  a  connective  tissue,  i.e.  a  mesoblastic  structure. 

Although  the  foetal  lens  like  that  of  the  adult  is  itself  non-vascular,  it  is  nevertheless 
■externally  freely  supplied  with  blood-capillaries,  which  form  a  vascular  tunic  completely 
surrounding  it  outside  the  capsule.  These  caxjillaries  are  supplied  by  a  branch  of  the  artcrla 
centralis  retina;  which  passes  forwards  tkrough  the  centre  of  the  vitreous  humour ;  in 
front,  at  the  margin  of  the  pupil,  they  come  into  continuity  v/ith  the  vessels  of  the  iris.  The 
most  anterior  part  of  this  vascular  tunic  forms  a  membrane  which  closes  the  aperture  of  the 
pupil  in  the  middle  periods  of  foetal  life.  In  the  human  eye  the  w^hole  tunic,  together  with 
the  artery  which  supi^lies  its  vessels,  becomes  atrophied  and  is  lost  sight  of  before  birth,  but 
in  some  animals  the^y^^^iZZa?-?/  moiibrane  remains  apparent  for  a  few  daj'S  after  biith. 

The  vitreoTis  iLumour  appears  to  be  fornied  from  the  mesoblastic  tissue  which 
has  passed  in  between  the  lens  and  the  inner  layer  of  the  optic  cup  by  a  gradual 
formation  of  a  larire  quantity  of  ground-substance,  whilst  the  cells  of  the  tissue 
almost  entirely  disappear.  The  development  of  the  hyaloid  membrane  has  not  been 
fully  traced  out,  and  the  same  may  be  said  with  regard  to  the  zonule  of  Zinn.  They 
are  probably  both  formed  by  part  of  the  same  mesoblast  as  forms  the  vitreous  humour 
(Lieberkiihn,  Angelucci). 

The  corneo-sclerotic  coat,  the  choroid  coat,  and  the  iris  are  all  derived  from  the 
mesoblast  surrounding  the  optic  cup. 

The  corneal  epithelium  is  a  portion  of  the  external  epiblast,  which  originally 
rests  against  the  front  of  the  lens  rudiment.  The  substantia  propria  comeae 
first  appears  in  the  chick  as  a  thin  homogeneous  layer  lying  immediately  within 

1  Kessler,  however,  looks  upon  this  homogeneous  deposit  as  being  alBO  a  cuticular  deposit  formed  by 
the  epithelial  cells. 


DEVELOPMENT    OF    THE    EYE. 


this  epithelium.  Into  this  homogeneous  layer  mesoblast  cells  pass  fi'om  the  margin^ 
greatly  thickening  it  and  producing  eventually  the  regular  layers  of  fibrous  tissue! 
which  are  characteristic  of  the  cornea.  Xo  cells  pass  into  the  most  anterior  or 
into  the  most  posterior  stratum,  which  remain  homogeneous  (anterior  and  posterior 

Fig.     103.  — •  Horizontal    sectioit 

THROUGH    THE    EYE    OF  AN    IJIBRYO 
RABBIT    OF    18    DAYS.  ^o  /J£j3j. 

liker.) 

0,  optic  nerve  ;  p,  hexagonal  pig- 
ment layei-  ;  r,  retina  ;  re,  ciliaiy 
part  of  the  retina  ;  x>\  foi-epart  of  the 
Optic  cuj)  (rudiment  of  the  iris  pig- 
mfent)  ;  g,  vitreous,  shrunk  away  from 
the  retina,  except  where  the  vessels 
from  the  arteria  centralis  retinje  enter 
it ;  i,  iris  ;  mj),  membrana  pupillaris  ; 
c,  cornea  with  epithelium  e  ;  pp,  pta, 
palpebrte  ;  I,  lens  ;  V,  lens  epithe- 
lium :  /,  sclerotic  ;  7ii,  recti  muscles. 

homogeneous  lamellae  of  Bow- 
man). The  epithelium  of  the 
posterior  homogeneous  lamella, 
or  membrane  of  Descemet,  is 
derived  from  mesoblast  cells 
which  grow  in  like  the  cor- 
neal corpuscles  from  the  mar- 
gin and  spread  themselves 
over  the  posterior  surface  of 
the  cornea,  thus  separating 
this  from  the  iris  and  anterior 
surface  of  the  lens.  For  a 
long  while,  however,  there  is  no  anterior  chamber ;  this  eventually  appears  as  a 
deft-like  space  between  the  cornea  and  the  structures  immediately  behind  it. 

In  mammals,  all  the  above  stages  of  formation  have  not  been  described.  A 
complete  layer  of  mesoblast  is  early  visible  lying  between  the  corneal  epiblast  and 
the  lens  epiblast,  and  continuous  around  the  margin  of  the  lens  with  the  mesoblast 
of  the  vitreous  chamber.  In  this  mesoblast  a  cleft  makes  its  appearance,  separating 
it  into  two  parts,  one  of  which  adheres  to  the  corneal  epiblast,  where  it  forms  the 
substance  of  the  cornea,  the  other  to  the  lens  capsule  forming  the  pupillary  membrane. 
This  cleft  is  the  rudiment  of  the  anterior  chamber.  It  does  not  become  actually 
distended  with  fluid  until  a  short  time  before  birth  (Kolliker). 

The  sclerotic  is  formed  entirely  from  mesoblast  around  the  optic  cup,  probably 
continuous  with  that  which  forms  the  cornea,  although  it  is  only  later  that  the 
cornea  and  sclerotic  come  to  be  completely  amalgamated. 

The  choroidjcoat  is  formed  from  the  mgsoblast  which  is  immediately  in  contact 
Avith  the  outer  layer  of  the  optic  cup,  and  tEeTTonVard  growth  of  the  middle  tunic 
closely  follows  that  of  the  margin  of  the  cup.  The  latter  ceases  at  first  at  the 
margin  of  the  lens,  but  subsequently  grows  forwards  over  the  front  of  the  lens  as  a 
thin  double  layer,  which  is  closely  covered  externally  with  a  continuation  of  the 
choroidal  mesoblast.  This  is  the  iris,  over  the  back  of  which  both  the  layers  of  the 
cup-margin  eventually  acquire  pigment  and  remain  permanently  as  the  uvea.  The 
ciliary  body  is  formed  by  a  kind  of  hypertrophy  of  the  optic  cup,'  which  developes 
radial  folds,  enclosing  thin  portions  of  mesoblastic  choroidal  tissue,  in  which,  as  in 
the  rest  of  the  choroid,  numerous  blood-vessels  and  branched  pigment-cells  become 
formed. 


DEVELOPMENT   OF   THE    EAR.  89 

Accessoxy  structures. — Tlie  eyelids  make  their  appearance  gradually  as  folds 
of  integument,  subsequently  to  the  formation  of  the  eyeball  (fi<^.  103).  About  the 
third  month  of  foetal  life  the  two  folds,  one  forming  the  upper  and  the  other  the 
lower  lid,  meet  and  unite  by  a  gi-owth  together  of  the  epithelium  at  the  margins  of 
the  folds,  so  as  to  cut  off  the  conjunctival  sac  from  the  exterior.  A  short  time 
before  birth  they  again  become  disunited. 

A  third  fold  (of  the  conjunctiva)  appears  at  the  inner  canthus,  and  in  many 
vertebrates  developos  into  a  well-marked  third  eyelid,  the  memhrana  nicUlans.  In 
man  it  remains  rudimentary,  forming  the  plica  semilunaris. 

The  glands,  hairs,  and  other  structures  belonging  to  the  eyelids,  are  deve- 
loped in  the  same  way  as  the  oorresponding  structures  in  the  rest  of  the 
integument. 

The  lachrymal  gland  is  developed  in  the  third  month  as  a  number  of  out- 
growths from  the  deeper  layer  of  the  epithelium,  at  the  upper  and  outer  part  of  the 
conjunctival  sac.  The  outgrowths  are  at  first  solid,  and  branch  into  the  surrounding 
connective  tissue  as  with  other  racemose  glands,  subsequently  becoming  hollowed 
out  and  differentiated  into  ducts  and  acini. 

The  lachrymal  canals  and  ducts  are  usually  described  as  being  directly 
developed  by  the  enclosure  of  the  fissure  which  separates  the  lateral  nasal  process 
from  the  maxillary  process  (see  Development  of  Nose,  p.  95,  and  figs.  Ill,  112), 
and  which  passes  in  the  early  embryo  from  the  eye  to  the  upper  part  of  the  naso- 
buccal  cavity  (lachrymal  fissure).  But  it  has  been  shown,  chiefly  by  the  researches 
of  Born,  that  in  most  animals  the  canal  is  at  first  formed  as  a  thickening  of  the 
rete  mucosum  of  the  epidermis,  which  sinks  into  the  corium  along  the  line  of  that 
fissure.  The  thickening  subsequently  becomes  separated  from  the  rest  of  the 
epidermis,  and  hollowed  out  to  form  an  epithelial  tube,  which  leads  from  the 
conjunctiva  into  the  nasal  cavity. 

The  bifurcation  of  the  duct  where  it  opens  on  the  conjunctiva  is  produced,  according  to 
Ewetsky,  by  a  broadening  out  of  the  epithelial  cord  at  the  inner  canthus,  and  its  subsequent 
separation  into  two  parts  by  an  ingrowth,  of  connective  tissue  in  its  middle,  the  two  parts 
developing  into  the  upper  and  lower  lachrymal  canals. 


DEVELOPMENT     OF     THE     EAH. 

The  essential  part  of  the  ear,  viz.,  the  epithelial  lining  of  the  labyrinth,  is 
developed  in  much  the  same  way  as  the  crystalline  lens,  as  an  invagination  of  the 
external  epiblast,  which  at  first  appears  as  a  pit  of  thickened  epithelium  {auditory 
jot7,  fig.  104,  A.),  but  is^radually  converted  by  a  growing  together  of  the  margins  of 
the  pit  into  a  hollow  island  of  epiblast,  the  auditory  or  otic  vesicle  (&g.  104,  B). 
This  process  occurs  somewhat  after  the  fonnation  of  the  eye  is  laid,  and  at  quite  a 
different  part  of  the  head,  viz.,  on  either  side  of  the  hind-brain  just  over  the  upper 
end  of  the  first  post-oral  visceral  cleft.  The  vesicle  comes  at  first  into  close  contact 
with  the  hind-brain,  except  where  the  gangHonic  rudiment  of  the  auditory  nerve 
projects  between  them,  but  it  subsequently  becomes  entirely  surrounded  by  meso- 
blast,  which  separates  it  from  both  the  neural  and  external  epiblast. 

The  hind-brain  does  not  send  out  a  hollow  process  towards  the  otic  vesicle 
con*esponding  to  the  optic  processes  of  the  fore-brain,  but  the  _auditory  nerve 
developes  from  a  solid  outgrowth  of  the  neural  crest  in  the  same  way  as  the 
posterior  roots  of  the  spinal  nerves  and  parts  of  many  other  of  the  cranial  nerves 
(see  p.  78). 

The  otic_vesiclejs  at  first  flask-shaped,  with  the  somewhat  elongated  mouth  of 
the  flask  directed  externalTy  towar3s  the  original  point  of  connection  with  the 


90 


DEVELOPMENT    OF    THE    EAE. 


exterior.  In  elasmobranch  fishes  this  connection  is  never  closed,  but  remains 
throughout  Hfe  in  the  form  of  a  small  duct-like  tube  which  passes  up  through  the 
cranial  wall  and  opens  on  the  epidermis.  In  other  vertebrates  the  connectjon  with 
the  exterior  becomes  closed — in  the  chick  during  the  third  day — and'what  remains 


Fig.    104.— Sections    through    region    op   the   hind-brain   of  human   embryos,  showing   three 

STAGES    IN    the    DEVELOPMENT    OF   THE    OTIC   VESICLE. 

A,  auditory  pits  ;  B,  simple  auditory  vesicles  ;  C,  auditory  vesicles  beginning  to  be  fashioned  into 
parts  of  the  membranous  labyrinth  ;  ot,  otic  vesicles  ;  rl,  recessus  labyrinth!  ;  v,  ventral  surface  of 
hind  brain  ;  r,  its  roof ;  /,  lateral  recess. 

Fig.  105.- — Outline  op  the  right  labyrinth  of  a  SJ 

WEEKS     human     EMBRYO,    TO    SHOW    ITS     RKLATIONS    TO 
THE   PARTS    OF    THE    AUDITORY    NERVE.      (W.   HlS,  jllU.) 

a.h,  section  of  hind  brain  ;  g.v,  ganglion  vestibuli  in 
contact  with  the  upper  part  of  the  labyrinth  ;  g.c,  ganglion 
cochleae  in  contact  with  the  lower  part.  The  fibres  of  the 
corresponding  parts  of  the  auditory  nerve  which  have  grown 
from  these  ganglia  into  the  hind  brain,  are  seen  to  cross 
one  another  ;  /,  facial  nerve  ;  r.l,  recessus  labyrinthi. 

of  the  original  mouth,  or  canal  of  connection 
with  the  exterior,  is  visible  as  a  distinct  but 
small  process  from  t"fe~Trppef  and  iiiner  angle 
of  ~Be  vesicle,  and  is  known  as  the  recess  of 
fhe-'Iadyrinth  (fig.  104  c,  r.l).  Evenlually^  it 
dereWJ)es'"mto  a  long  epithelial  tube,  which 
passes  through  the  petrous  bone,  with  an  expanded  end  lying  within  the  skull 
underneath  the  dura  mater:  "This  tube  and  its  expanded  termination  forpa  respec- 
tively the  endoli/mphat'ic  canal  and  sccccule  (fig.  106). 

In  the  meantime  the  auditory^vesicle  becomes  elongated  and  begms  to  be 
irregidar.  Its  ventral  end  projects  as  a  distinct  hollow  process,  at  first  straight, 
but^soon  becoming  curved  ;  this  is  the  rudiment  of  the  epithehal  amdgfjhe  cochlea. 


DK\  i:l()|'mknt  of  the  ear. 


91 


OthgjiJldlfflaLprojections  appear  near  the  dorsal  end  of  the  vesicle  ;  these  form 
the  two  superior  semicircular  canals  ;  the  horizontal  camd-ttppeftm-a  litj^^eJatAj.-. 

The  mode  of  formation  of  the  canals  is  somewhat  pocnliar.  They  first  appear  as  liattcneil 
•semicircular  hollow  pnitnisious  <>f  tlic  wall  of  the  vesicle.  Their  sides  then  come  to^.'-cither 
and  coalesce,  except  near  the  circumf('rencc  of  the  semicircle,  which  now  forms  a  tube  con- 
nectetl  at  both  ends  with  the  vesicle.  Subseciuently  a  separation  or  lireach  of  continuity 
occurs  over  the  area  of  coalescence,  so  that  the  rest  of  the  tube  is  free.  One  of  the  endH 
becomes  dilated  into  an  ampulla  and  connected  with  a  branch  of  the  auditory  nerve. 

"Whilst  these  processes  are  occnrviiiu'  at  the  dorsal  and  ventral  ends  of  tlie  now 
elongated  vesicle,  a  fold,  or  constriction,  of  the  wall  is  beginning'  to  make  its 
appearance  about  the  middle,  and  thus  the  posterior  part  which  is  connected  with  the 
semicircular  canals  becomes  gradually  separated  (as  the  utricle)  from  the  anterior 
part,  which  forms  the  saccule,  and  is  connected  with  the  cochlea.  This  fold  extends 
into  the  beginning  of  the  recess  of  the  labyrinth,  and  separates  it  longitudinally  for  a 


p.s.c. 


Fig.  106. — Stages  in  the  development  op  the  membranous  labyrinth.     (W.  His,  juu.) 

A.  Left  labyrinth  of  a  human  embryo  of  about  four  weeks,  viewed  from  the  outer  side,  v,  vestibu- 
lar part  ;  c,  cochlear  part  ;  r.l,  recessus  labyrinthi  (aquaaductus  vestibuli). 

B.  Left  labyrinth  with  parts  of  the  facial  and  auditoiy  nerves  of  a  human  embryo  of  about  4^ 
weeks.  6,  surface  of  the  hind  brain;  m,  utricular  ;  s,  saccular  part  of  labyrinth;  a.s.c,  x>-S.c., 
c.s  c,  rudimentary  folds  representing  the  two  vertical  and  the  horizontal  semicircular  canals  ;  r.l,  upper 
part  of  recessus  labjTinthi  becoming  enlarged  into  the  endolymphatic  saccule  ;  c.c,  rudiment  of  cochlea  ; 
n.v,  vestibular  branch  of  auditoiy  nerve  ;  g.v,  vestibular  ganglion  (ganglion  of  Scarpa)  ;  g.c,  cochlear 
ganglion  ;  n.f,  facial  nerve,  with  geniculate  ganglion,  r/.g. 

C.  Left  labyrinth  of  a  human  embryo  of  about  five  weeks,  viewed  from  without  and  below.  Letter- 
ing as  before.  The  horizontal  canal  is  still  only  a  fold.  The  ampulhe  are  beginning  to  be  visible  on  the 
two  vertical  canals. 


short  distance  into  two  tubes,  one  of  which  opens  into  the  utricle,  and  the  other 
into  the  saccule,  forming  the  only  permanent  means  of  communication  between 
their  contents.  Another  fold,  or  constriction,  appears  presently,  somewhat  lower 
down,  and  converts  the  connection  between  the  saccule  and  the  cochlea  rudiment 
into  the  narrow  duct  of  Hensen  {canalls  re-unims). 

In  the  meantime  the  cochlea-rudiment  at  the  ventral  end  of  the  now  labyrinthic 
vesicle,  becomes  elongated  into  a  tube,  which,  as  it  grows,  becomes  coiled  upon 
itself  in  such  a  manner  as  to  produce  the  spiral  structure  of  this  pai-t  of  the  auditory 


92 


DEVELOPMENT    OF   THE   EAR. 


Fig.  107. — Transverse   and    slightly   oblique   section   of  the  head  or  a  fcetal  sheep,  in  the 
REGION  of  the  HIND  BRAIN.     (From  Foster  and  Balfoui' after  Boettcher. ) 

HE,  innei  surface  of  the  thickened  vails  of  the  hind  brain  ;  RV,  recess  of  the  vestibule  ;  TB, 
commencing  vertical  semicircular  canal ;  CC,  canal  of  the  cochlea  ;  GC,  cochlear  ganglion  of  the  right 
side;  on  the  left  side.  G',  the  ganglion,  and  N,  the  auditory  nerve  connected  ^vith  the  hind  brain. 


Fig.    108. — Transverse  section  of 

THE     HEAD    OF    A   FOETAL    SHEEP    OF 
four-fifths  of  an  inch  in  LENGTH. 

(From    Foster    and   Balfour    after 
Boettcher. ) 

EV,  recessus  vestibuli ;  VB,  vertical 
semicircular  canal  ;  CC,  cochlear 
canal  ;  G,  cochlear  ganglion  ;  HB, 
horizontal  canal. 

organ.  This  coiling,  however, 
only  occurs  in  mammals  ;  in 
birds,  the  cochlea  is  a  short 
straight  blind  tube. 

All  these  parts  of  the  laby- 
rinth are,  when  first_  formed, 
simple  epithelial  tubes  sur- 
rouiicTed  by  and  imbedded  in 
embryonic'  lionnective  tissue. 
As  development  proceeds,  and 
the  skull  begins  to  form,  a 
cartilaginous  capsule  becomes 
developed  around  the  several 
parts  of  the  labyrinth,  and 
this  at  length  becomes  ossified. 
The  cartilaginous  capsule  does 
not  closely  invest  tlie  epithelial  structures  ;  they  are  immediately  surrounded  by 
embryonic  connective  tissue,  which  forms  an  internal  periosteal  lining  to  the  capsiile 
and  a  special  covering  to  the  epithelial  tube.  These  two  connective  tissue  membranes 
are  everywhere  separated  from  one  another  by  gelatinous  connective  tissue,  composed 


EXTERNAL    AND    MIDDLE    EAR.  93 

of  semi-fluid  oround  substance  and  branching  corpuscles,  except  along  one  border, 
where  they  are  in  continuity.  But  in  the  cochlea  the  gelatinous  tissue  is  above  and 
below  the  epithelial  tube,  the  place  of  the  modiolus  being  occupied  by  embryonic 
tissue  which  is  not  gelatinous,  and  is  connected  with  that  lining  the  capsule  by 
similar  non-gelatinous  tissue  separating  the  turns  of  the  cochlea  from  one  another, 
and  also  running  in  the  position  of  the  future  spiral  lamina. 

The  bone,  which  is  formed  by  03sification  of  the  cartilaginous  capsule,  is  of  a  spongj 
nature,  but  it  becomes  coated  internally  by  layers  of  compact  bone  deposited  by  the  periosteal 
lining.  The  modiolus  and  septa  of  the  cochlea,  as  well  as  the  osseous  spiral  lamina,  are 
formed  wholly  in  connective  tissue  without  any  preformation  in  cartilage. 

The  perilymphatic  spaces  throughout  the  whole  labyrinth  are  produced  by  a 
gradual  vacuolation  and  disappearance  of  the  gelatinous  tissue  which  surrounds  the 
membranous  labyrinth.  In  the  cochlea  this  conversion  into  perilymph  begins  in  the 
proximal  turn  of  the  spiral  and  extends  hence  towards  the  distal  end.  It  is  only 
with  the  development  of  these  perilymph-spaces  (scalae)  that  the  cochlear  tube, 
which  was  previously  oval  in  section,  acquires  the  characteristic  triangular  section 
which  we  see  in  the  fully-formed  organ. 

The  auditory  nerve  is  large  and  early  becomes  separated  into  its  two  main  divisions,  vesti- 
bular and  cochlear.  Each  division  has  a  large  ganglion  upon  it  (fig.  105),  which  extends 
to  the  anterior  wall  of  the  epithelial  vesicle,  and  as  the  ventral  end  of  the  vesicle  elongates 
and  assumes  the  spiral  disposition,  the  cochlear  nerve  and  ganglion  extend  along  with  it  and 
take  the  same  coiled  or  spiral  form. 

The  cells  which  form  the  wall  of  the  epithelial  tube  become  variously  modified  in  different 
parts  of  the  labyrinth  to  produce  the  characteristic  structures  which  there  occur,  viz.  :  the 
haii"-cells,  the  rods  of  Corti,  the  sustentacular  cells  of  Deiters  and  the  epithelium  lining  the 
labyrinth.  The  membrana  tectoria  appears  as  a  cuticular  deposit  over  the  columnar  cells 
which  are  becoming  developed  into  the  organ  of  Corti. 


ACCESSORY    PARTS    OF    THE    ORGAN     OF    HEARING.      EXTERNAL    AND     MIDDLE     EAR. 

While  the  epithelium  of  the  internal  ear  is  formed  by  an  involution  of  cutaneous 
epiblast  in  the  manner  which  has  just  been  explained,  the  middle  ear  with  the 
Eustachian  tube,  and  the  external  auditory  meatus  with  the  pinna  are  formed  from 
the  remains  of  the  first  visceral  cleft", 'and  from  the  parts  of  the  mandibular  and 
hyoideaB!"afcTi^"wIncF  immediaEely  iDound  the  cleft.  This  cleft  at  an  early  period 
forms  an  almost  complete  communication  between  the  pharynx  and  the  exterior,^  but 
the  broad  cleft  becomes  gradually  converted  into  a  flattened  tube,  and  this  is  presently 
found  to  be  closed,  both  by  the  epiblast  and  hypoblast,  which  are  from  the  first 
in  contact  at  the  bottom  of  the  cleft,  and  also  by  an  ingrowth  of  mesoblast,  the 
rudiment  of  the  membrana  tympani  being  thus  formed.  There  is  at  first  no  enlarge- 
ment of  the  flattened  tube  to  represent  the  tympanic  cavity,  and  the  ossicles  are 
developed  not  within,  but  altogether  outside  the  tube,  in  a  mass  of  gelatinous  con- 
nective tissue,  which  is  continuous  with  that  forming  the  embryonic  membrana 
tympani  ;  they  are  formed  for  the  most  part  by  ossification  of  parts  of  the  carti- 
laginous bars,  which  extend  from  the  otic  capsule  into  the  mandibular  and  hyoidean 
visceral  arches  (see  Development  of  Skeleton).  As  the  tympanic  cavity  becomes 
formed  by  a  gradual  enlargement  of  the  blind  end  of  the  closed  hyomandibular  cleft, 
the  gelatinous  tissue  retires  before  it,  and  as  this  tissue  disappears,  the  ossicles  and 
the  chorda  tympani  which  were  previously  entirely  enveloped  by  it,  are  left  projecting 
into  the  tympanic  cavity,  covered  only  by  thin  mucous  membrane.  The  process  of 
formation  of  that  cavity  is  not,  in  fact,  completed  until  after  birth,  when  air  becomes 
admitted  into  it  thi'ough  the  Eustachian  tube. 

*   Vide  footnote  on  p.  102, 


94 


DEVELOPMENT    OF    THE    EAR. 


The  embryonic  tympanic  membrane  is  at  first  close  to  the  exterior,  the  external 
meatus  being  scarcely  existent,  although  the  several  parts  of  the  external  ear  are  very 


Fig.   109. — Sketches  showing   the   gradual   development   of  the   parts  op  the  external  ear 

FROM    PROMINENCES    UPON    THE    MANDIBULAR    AND     HTOIDEAN    VISCERAL    ARCHES.       (His. )       Variously 

magnified. 

F  is  an  outline  sketch  showing  the  several  parts  of  a  well- developed  adult  ear,  frd  natural  size. 

1,  2,  prominences  on  the  mandibular  arch;  3,  prominence  between  the  two  arches,  immediately 
over  the  cleft,  prolonged  posteriorly  into  c,  behind  the  hyoidean  arch';  4,  5,  and  6,  prominences  on  the 
hyoidean  arch  ;  L,  in  B,  otic  vesicle  (seen  also  in  A)  ;  K,  lower  jaw. 

Of  the  prominences  enumerated  1  forms  the  tragus  ;  2,  3,  and  3c,  the  helix  ;  4,  the  antihelix ; 
5,  the  antitragus  ;  and  6,  the  lobule  {vide  F). 


DEVELOPMENT    OF    THE    NOSE. 


95 


early  distinguishable  as  slight  protnberaaces  upon  the  margins  of  the  shallow  cleft- 
like (lepivssion  which  is  all  that  represents  the  meatus  at  this  stage  (fig.  1 10,  a).  But 
as  the  boily  wall  becnmes  thicker,  the  cleft  becomes  deei»ened  and  more  tubular,  and 
the  protuberauces  upon  the  mandil»ular  and  hyoideau  arches  Itecotne  gradually  so 
tniusformed  and  arranged  amuud  the  external  orifice  as  to  Ije  recognizable  as  the 
several  parts  of  the  future  pinna.  The  transformations  may  readily  be  understood 
from  the  study  of  the  accompanying  series  of  sketches  from  IIi.s,  which  show  these 
piirts  in  gradually  advancing  stages  in  the  human  embryo  (fig.  lu'jj, 

DEVELOPMENT     OP     THE     NOSE. 

The  olfactory  organ  arises  in  all  verte-brates  at  an  early  period  of  embryonic  life 
AS  a  depression  of  external  epiblust  {olfaciory  pit)  on  eithej-  side  of  the  fore-brain. 
The  epi  blast  in  this  regioiiDecomes  thickened,  forming  an  olfactorij  area,  and  a  de- 


iJt-.* 


-pr.gloK 


Fig.    110. fROFILE  riKW  OF  TffE  HEAD  OF  A 

HUMAN  EMBRYO  OF   NEARLY   FOrR  WEEKS.       (His.) 

olf,'  olfactory  depression  passing  posteriorly 
into  a  deep  pit,  the  rudiment  of  Jacobson's 
organ  ;  mi,  maxillary  process  :  »in,  mandibular 
arch  ;  liy,  hyoidean  arch  ;  6r',  hifi,  first  and 
second  branchial  arches. 


Fig.   111. — Head  of  ax  embryo   more   adtascei^ 
IX    bevelopmejtt    thah    that    suowx    is    fig. 

110,   FKOM  BEFORE.       (His.  ) 

'pr.f/lohf  globular  extremity  of  the  mesial  nasal 
process.     The  other  letters  as  in  fig.  111. 


pression  then  forms  in  this  area  surrounded  by  a  raised  margin  (figs.  96,  97,  olf). 
The  depression  soon  appeal's  pyriform,  the  smaller  end  extending  as  a  groove  towards 
the  storaodoeum  or  buccal  invagination  (see  fig,  110,  olf) ;  near  this  end  a  special 
pit  is  early  visible,  and  becomes  developed  into  Jacobson's  organ. 

The  thickened  b^oundaries  of  each  ulfaituiy  pit  and  g.roove  ar?  formed  by  the  so- 
called  imsial  and  lateral  nasal  processes  (tigs.  Ill,  112).  The  mesial  nasal  processes 
are  iinT^d' at 'their  base  by  a  depressed  median  pare  of  the  fronto-nasal  process,  but 
are  at  first  separated  below,  where  they  terminate  in  distinct  tubercles,  termed  by 
His  the  (/lobular  processes.  As  development  proceeds  they  extend  backwards  along 
the  roof  of  the  emb^onic^mouth,  forming  the  nasal  lamTnTT.  Eventually  the 
globular  processes  coalesce  in  the  middle  Hne'tb  form^  the  fntermaxillary  process  and 
the  middle  part  of  the  lip,  while  from  the  depressed  surface  between  them  the 
lower  part  of  the  nasal  septum  and  the  philtnmi  are  fonued,  and  by  a  coalescence  of 
the  nasal  laminae  the  rest  of  the  nasal  septum  is  produced.  In  rodents  a  notch  leads 
from  the  nasal  septum  through  the  upper  lip  to  the  mouth,  and  represents  an  im- 
perfect union  of  the  globular  processes. 

Above  the  depressed  surface  just  referred  to,  is  a  triangular  part  of  the  fronto- 
nasal process  whidr'fofmg' aii  aiigle^wriEE  it.    This  angle  eventually  becomes  the 


96 


DEVELOPMENT    OF    THE    NOSE. 


-^nalcR 


frv.vn 


jjr.  o.l. 


Fig.  112. — Head  of  ax  ejibryo  still  more  advanced  in  development.     (His.) 

A,  from  above  ;  B,  roof  of  mouth  after  removal  of  lower  jaw  ;  i.m,  placed  on  the  fronto-nasal 
process  and  just  above  its  intermediate  depressed  part;  l.n.pr,  lateral  nasal  process;  m.n.pr,  mesial 
nasal  process  ;  the  other  letters  as  before.  The  nasal  lamince  of  the  globular  processes  and  the 
palatine  projections  of  the  maxillary  processes  are  seen  in  B. 


fy 
.# 


w 

V 
J'' 


T'ig.  113. — Head  of  an  embryo  of  about   seven 
weeks.     (His.) 

The  external  nasal  processes  have  united  with  the 
maxillary  and  globular  processes  to  shut  off  the 
■olfactory  pit  from  the  orifice  of  the  mouth. 


Fig.  114. — Head  of  an  embryo  more  advanced 
IN   development,  with    the    parts  of  the 

NOSE  AND  MOUTH  BEGINNING    TO  ASSUME  THEIR 
PERMANENT    RELATIONSHIPS.       (His.) 


point  ofjhe  nose,  and  the  triangular  surface  above  it  the  bridge  ;  the  alse  nasi  are 
formed  by  the  lateral  nasal  jirocesses.  These  pfo'cfe'ses'a^^  prominent  than  the 
mesial  (fig,  1]  2).~They  curve  round  the  olfactory  depressions,  and  meet  the  maxillaiy. 


DEVELOPMENT    OP    THE    XOSE. 


97 


processes  ;  between  tlif  two  pnxu'ssi's  (lateral  nasal  ami  m  ixillary)  th(!  lachrymal 
<i;;ro()ve  parses  from  the  eye  to  the  nose  (ti^s.  HI,  112).  'I'hc  maxillary  pritecsses 
also  abut  in  front  against  the  outwardly  curving  ends  of  the  processus  globulares, 
which  together  form  as  just  mentioned,  an  intermaxillary  process,  and  the  three 
eventually  coalesce  to  form  the  upper  l)oundaiy  of  tlu^  mouth,  which  is  thus  shut 
off  from  the  anterior  (irifice  of  the  nasal  fossa)  (tig.  11;)).       But  further  back  the 


Fig.    ll'l.— OUTLINK    OF    A    TRANSVKHSK    VKKTICAI.    SECTION    TilKOL'GH    THK    XOSK    AND    UI'I'KR    JAWS    OF    A 
SHKKI-'S    KMBKYO    WITH    OPEN    I'AI.ATE.       (Froill   Kolliker.) 

The  lower  j;iw  ami  tongue  are  removed  ;  d,  dental  germs  ;  p,  the  palate  plates  approaching  each 
other  in  the  mitUlie  ;  /,  the  nasal  fossie  ;  c,  nasal  cartilage  ;  s,  septal  cartilage  ;  j,  the  two  organs  of 
Jacohson  with  their  cartilages  internally. 

olfactory  depressions,  which  are  now  developed  into  cleft-like  cavities,  and  are  in- 
creasing in  complexity  by  the  development  of  the  projections  which  are  to  form  the 
turbinate  bones,  are  still  freely  in  communication  with  the  buccal  cavity,  and  it  is 
only  by  the  growth  of  the  palatine  processes  of  the  maxilke  (fig.  115,jt;)  and  their 
coalescence  in  the  middle  line  with  one  another,  and  with  the  lower  part  of  the  nasal 
septum,  that  the  nasal  cavities  are  cut  off  from  the  mouth  and  from  one  another, 
and  now  only  open  posteriorly  into  the  upper  part  of  the  pharynx  by  the  posterior 
nares  {(•Iioamc). 

The  median  or  septal  part  of  the  external  nose,  with  its  columella  below,  is 
formed,  as'aljove'statFd^'tif'the  co'tdesced  mesial^n  processes,  the  alte  nasi  being 
developed  from  the  lateral  nasal  processes.  The  sepjum  is  at  first  broad  and  de- 
pressed, so  that  the  nostrils  are  widely  separated  from  one  another  (fig.  114),  a  con- 
dition which  remains  to  a  certain  extent  permanent  amongst  some  of  the  dark  races 
of  mankind. 

From  the  above  description  it  will  be  seen  that  the  olfactory  organs  are  at  first  altoj^ether 
distinct  from  the  mouth,  that  they  subsequently  pass  backwards,  as  >;Tooves.  deepening  into 
distinct  clefts,  along  the  roof  of  the  mouth  and  forming  in  fact  the  upper  part  of  the  embryonic 
buccal  cavity,  and  that  finally  they  are  again  gradually  separated  from  that  cavity  Ijj-  the 
growth  of  a  horizontal  septum  to  form  at  fir.st  the  hard  and  afterwards  the  soft  palate. 

The  median  union  of  the  palate  begins  in  front  about  the  eighth  week  and  reaches  the 
back  i^arc  and  is  completed  about  the  tenth  week.  Imperfect  coalescence  of  these  parts  pro- 
duces the  malformations  of  hare-lip  and  cleft  palate  in  their  various  degrees.  Usually,  how- 
ever, in  man  the  coalescence  is  completed  at  a  comparatively  early  period  of  foetal  life, 
although  a  vestige  of  the  original  separation  may  be  found  in  front  at  the  junction  of  the 
maxillary  processes  with  the  coalesced  globular  processes  (intermaxillary),  as  the  naso-palatine 
canal  or  incisor  foramen,  which  is  occupied  by  connective  tissue,  blood-vessels,  and  a  branch 
of  the  fifth  nerve.  In  many  mammals,  however,  an  actual  communication  remains  through- 
out life  between  the  nostrils  and  mouth  in  this  situation. 

The  organ  of  .Jacobson  is  early  visible  on  either  side  of  the  nasal  septum  at  its  lower  part 
in  the  form  of  a  narrow  tube,  oval  in  section,  running  horizontally  in  the  substance  of  the 
septum  and  opening  anteriorly  near  the  upper  orifice  of  the  naso-palatine  canal.  When  the 
cartilage  of  the  septum  becomes  formed,  a  special  curved  plate  of  cartilage  is  seen  partially 
enclosing  this  organ  ;  but  both  the  organ  itself  and  the  cartilage  are  less  con.spicuous  in  man 
than  in  most  mammals.    According  to  Geirenbaiu'  the  rudiment  which  has  been  described  in 


98  RECENT    LITERATURE. 

the  human,  embryo  as  the  organ  of  Jacobson  is  not  really  that  structure,  but  represents  a 
special  g-land  which  occurs  in  some  lemurs  in  the  lower  part  of  the  nasal  septum. 

The  epiblast  of  the  olfactory  area  early  becomes  thickened,  and  resembles  in  structure  the 
neural  epiblast  (His).  As  in  the  latter,  some  of  the  cells  (neuroblasts)  become  pyriform  and 
nerve-fibre  processes  grow  out  from  them,  whilst  the  remainder  form  long  sustentacular 
columns,  which  partially  anastomose  to  form  a  spongework.  The  neuroblasts  subsequently 
pass  out  towards  the  olfactory  lobe  as  ah-eady  described  (p.  81). 

All  the  complexities  of  the  nasal  fossae  (and  they  are  far  more  complex  and  labyrinthic  in 
many  animals  than  in  man)  are  produced  by  folds  and  outgrowths  of  the  original  simple 
depressions,  and  the  thickened  epithelium  of  these  depressions  extends  over  all  parts  of  the 
cavities  which  are  thus  formed.  But  it  is  only  in  the  upper  part  of  the  nasal  f ossEe  that  the 
connexion  by  nerve-fibres  with  the  olfactory  lobe  becomes  established,  and  it  is  in  this  part 
only  that  the  true  sense-epithelium  becomes  developed.  In  the  lower,  or  respiratory  part 
of  the  f  ossse  the  epithelium  remains  relatively  thin  and  becomes  ciliated. 


BECENT    LITERATURE. 

Eye  and  Nose. 

Born,  G-.,  Die  NasenTioIde  u.  der  Thrdnennasenr/anr/  der  amnioten  Wirhelthiere,  Morpt.  Jahrb., 
1879  u.  1883. 

Disss,  J.,  Die  Aushildung  der  Nasenhohle  nach  der  Geburt,  Arch.  f.  Anat.  u.  Physiol.,  Anat. 
Abth.  1889. 

Ewetzky,  Th.,  Zur  EntwicTcelungsgesckicTite  des  Thrdnennasenganges  leim  Menschen,  Archiv  f. 
Ophthalmol.,  Bd.  xxxiv.,  1888. 

G-ottschau,  Zur  Entioickelung  der  Sdugethierlinse,  Anat.  Anzeiger,  18S6. 

His,  W.,  AnatomiemenseUicherEmbryonen,  Leipzig,  1880 — 85  ;  Unsere  Eorperform,  tfcc,  Leip- 
zig, 1875  ;  Die  FormentwicJcdung  des  menschlichen  Vorderhirns,  Abhandl.  d.  k.  Sachsischen  Gesell- 
schaft,  1889. 

Keibel,  F.,  Zur  Entwickelung  des  Glaskorpers,  Arch.  f.  Anat.  u.  Phys.,  Anat.  Abth.,  1886. 

Kessler,  Zur  Entwickelung  des  Auges  der  Wirhelthiere,  Leipzig,  1877. 

KcUiker,  A.,  Zur  Entivickclung  des  Auges  und  Geruchsorganes  menschlicher  Emhryonen,  Fest- 
schrift der  Schweizer.  Universitat  Zurich  gewidmet,  Wiirzburg,  1883. 

Koranyi,  A.,  Beitrdge  zur  Entwickelung  der  KrystalUnse  lei  den  WirheUMeren,  Intemat.  Mo- 
natsschr.  fiir  Anatomie  u.  Physiologic,  1886. 

Legral,  Die  Nasenhohle  u.  der  Thrdnennasengang,  Morph.  Jahrb.,  1883. 

Ear. 

Baginsky,  B.,  Zur  Entwickelung  der  Gehorschnecke,  Verhandl.  d.  physiol.  Gesellschaft  za  Berlin, 
1885-86. 

Beard,  J.,  On  the  segmental  sense  organs  of  the  lateral  line  and  on  the  morphology  of  the  vertebrate 
auditory  organ,  Zool.  Anzeiger,  No.  161,  1884. 

Boettcher,  A.,  Ueber  Entwickelung  u.  Bau  des  Gehorlabyrinths,  Verhandl.  d.  kaiserl.  Leop. 
Carol.  Acad.,  Dresden,  Bd.  35. 

His,  "W.,  jun.,  Zur  Entwickelung sgeschichte  des  Aeustico-facialis-Gebietes  beim  Menschen.  Arch, 
f.  Anat.  u.  Physiol.,  Anat.  Abth.,  1889,  Supplement  Bd. 

Hoffmann,  C.  K.,  Ueber  die  Beziehung  der  ersten  Kiementasche  su  der  Anlage  der  Tuba  Eustachii 
und  des  Cavum  tympani,  Archiv  f.  mikrosk.  Anat.  xxiii.,  1884. 

Noorden,  v..  Die  Entwickelung  des  Labyrinths  bei  Knochenfischen,  Arch.  f.  Anat.  u.  Physiol., 
Anat.  Abth.,  1883. 

Rudinger,  N.,  Zur  Entwickehmg  der  hdutigen  Bogengdnge  der  inner  en  Ohres,  Sitzungsber.  d. 
Akademie  d.  Wissensch.  zu  Miinchen,  Bd.  xviii.,  1888. 

Tuttle,  Alb.  H.,  The  relation  of  the  external  m.eatus,  tympanum  and  Eustachian  tube  to  the  first 
visceral  cleft,  Proc.  American  Academy  of  Arts  and  Sciences,  1884. 

Vassaux,  Recherches  sur  les  premieres  pjhases  du  developpement  de  Vceil  chez  le  lapin  Arch 
a'ox)hthalm.,  1888. 


DEVELOPMENT    OF    THIC    MoLTIl.  <.JD 


DEVELOPMENT    OF    THE    ALIMENTARY    CANAL. 

The  early  development  of  the  primitive  alimentary  canal  has  already  been  briefly 
described  in  treating  of  the  first  formation  of  the  embryo  (pp.  :U,  :-j;")),  and  it  was  there 
explained  how  the  dipping  downwards  and  inwards  of  the  blastodermic  layers  on 
either  side  of  the  embryo  tends  to  separate  or  pinch  off  the  part  of  the  blastodermic 
vesicle  which  is  immediately  underneath  the  body  of  the  embryo  as  a  distinct  tube 
(mid-gut)  from  the  remainder  of  the  vesicle,  which  is  now  known  as  the  yolk-sac,  while 
at  the  same  time  similar  changes  occurring  in  ft'ont  and  behind  produce  the  blind 
anterior  and  posterior  extremities  of  the  tube  which  are  known  as  the  fore-  and  hind- 
gut  respectively.  Although  the  downfolding  in  question  eventually  involves  all  the 
layers  of  the  embryonic  blastoderm,  the  epiblast  and  the  part  of  the  mesoblast  which 
adheres  to  it,  and  together  with  it  forms  the  somatopleure,  do  nob  participate  in 
the  process  until  afoer  the  formation  of  the  amnion,  so  that  the  alimentary  canal  for 
some  time  after  its  formation  is  enclosed  only  by  the  liypbblast  and  Jis  adherent 
mesobkst~(^splanciinqpleure),  and  projects  freely  into  the  wide  coelom  or  space 
between  the  splanchnopleure  and  somatopleure.  The  mid;:gut  also  remains  for  a 
time  in  free  communication  with  the  yolk. sac,  although  the  communication  becomes 
gradually  narrowed  into  the  vitelline  duct.  As  the  somatopleure  afterwards  grows 
dowiTon  eilKer  side  of  the  alfmentafy"canal,  and  becomes  pinched  in  around  the 
vitelline  duct  and  stalk  of  the  allantois,  which  are  thus  united  into  the  umbilical 
cord,  that  part  of  the  coelom  which  is  within  the  body  and  around  the  alimentaiy 
canal  becomes  shut  off  as  the  pleuroperitoneal  cavity  from  the  remainder,  which  lies 
altogether  outside  the  body,  and  forms  the  cavity  of  the  false  amnion. 

Development  of  the  mouth  and  of  the  parts  iu  connection  with  it. — 
The  fore-guc  terminates  blindly  at  first  underneath  the  head  iu  the  region  of  the 

Fig.  116. — Froxtal  view  of   the  tpper  part  of  a  humax 

EMBRTO     OF    ABOUT     FIFTEEN'     DAYS,     RECOXSTRCCTED    FROM 
SERIAL    SECTIONS.       (His.)       *' 

The  pericardium  is  opened  to  show  the  heart  :  between  this 
and  the  fore-brain  is  seen  the  primitive  buccal  cavity.  A  de- 
scription of  this  figure  is  given  on  p.  138. 

mn. 

hind-brain,  and  the  notochord,  with  the  fore-  and  , 

.  o.a. 

mid-brain,  curve  downwards  over  the  blind  extre- 
mity, the  fore-brain  thus  causing  a  rounded  pro- 
minence in  fi'ont  of  and  ventral  to  the  extremity  of    '"• 
the  alimentary  tube  (see  fig.  4.5).    With  the  develop- 
ment  of  the  heart  another  prominence  becomes  "l^ 
formed  on  the  ventral  side  of  the  fore-gut,  a  little  ''•"• 
further  back.     Between  the  two  prominences,  the 
one  caused  by  the  projection  of  the  fore-brain  and 
the  other  of  the  heart,  a  wide,  shallow  pit  is  enclosed 
(fig.  lie),  at   the  bottom  of  which  the  ejDJblast 

which  lines  it  is  in  contact  with  the  hypoblast  of  thefore-gut,  and.  the -two-  layere 
fuse  to"  form  an  epithelial  raembnT.ne,  which  now  forms  a  septum  between  the 
primitive  buccal  epiblastic  involution  or  stomoda^um  and  the  fore-gut  (fig.  117, 
p.v.).  This  stage  is  met  with  in  the  human  embryo  before  the  twelfth  day  (His), 
in  the  rabbit  embryo  at  about  the  ninth  day  (Mihalkovics),  and  in  the  chick  on  the 
fourth  day. 

H  2 


100 


DEVELOPMENT    OF    THE    MOUTH. 


The  stomodseum  deepens  at  its  upper  and  anterior  part,  where  it  forms  a 
pocket-like  protrusion,  which  grows  a  certain  distance  into  the  angle  formed  by  the 
sharp  bend  which  the  hinder  part  of  the  fore-brain  now  makes  with  the  mid-brain. 


enoi. 


-t^i.v-. 


u.a-r 


^'^^ccZi,. 


Fig.  117. — Pkofile  view  of  a  human  embryo  of  about  15  days,  with  the  alimentary  canal 

SHOWN    IN    longitudinal    SECTION.       (His.) 

Fig.  118. — Similar  a'iew  of  a  somewhat  older  embryo.     (His.) 

1,  2,  -3,  4,  5,  are  opposite  the  respective  secondary  cerebral  vesicles  ;  from  the  side  of  the  fore-brain 
the  primary  optic  vesicle  is  seen  projecting  ;  ot.  otic  vesicle  ;  "p-v.,  septum  between  mouth  and  pharynx 
(primitive  velum)  ;  I,  commencing  liver  in  septum  transversum  ;  v,  vitelline  stalk  ;  all,  allantois  enclosed 
within  allantoic  stalk  ;  j  v.,  jugular  vein  ;  c.v.,  cardinal  vein  ;  s.r.,  sinus  venosus  within  septum  trans- 
versum ;  u.a.,  umbilical  (allnntoic)  artery;  l.u.v.,  left  umbilical  vein  ;  end.,  endothelial  tube  of  heart. 
The  sharp  curve  of  the  truidc  of  the  cmbi-yo  towards  the  yolk-sac  is  normal  at  this  stage. 

In  fig.  117  the  otic  vesicle  is  still  oiDen,  and  there  are  only  two  aortic  arches  ;  in  fig.  118  the  otic 
vesicle  is  clcsed  ;  there  are  now  five  aortic  arches.     The  primitive  velum  has  disijiijeared. 


Tliis.^gocket  (Eathke)  is  the  iM/PMllMW.  cmybri,  or  pituitary  involution  of  the  buccal 
epiblast,  and  comes  presently  into  connection  with  the  infundibular  protrusion  of  the 
neural  epiblast,  the  two  together  forming  the  pituitary  body  (see  p.  G8).  It  lies 
just  above  and  in  front  of  the  pharyngeal  septum. 

The  remains  of  this  septum  (when  it  has  become  broken  through  to  allow  of  a  communica- 
tion between  stomodgeum  and  fore-gut),  have  been  termed  the  2}rimitii-e  vehim,  hut  the  septum 
has  nothing  whatever  to  do  with  the  formation  of  the  permanent  velum  palati,  or  with  the 
isthmus  of  the  fauces.  The  plane  of  the  septum  forms  in  fact  an  angle  with  the  plane  of  the 
future  isthmus  faucium,  so  that  the  primitive  mouth  or  stomodEeum  does  not  by  any  means 
correspond  with  the  permanent  mouth.  In  fact  the  floor  of  the  mouth,  including  the  tongue, 
is  developed  hehind  the  septum,  and  therefore  in  connection  with  the  fore-gut  rather  than  with 
the  stomodEeum,  whereas  the  uppermost  part  of  the  pharynX;  including  the  choanas,  is  in  front 
of  the  septum,  and  therefore  belongs  to  the  stoinoda3um. 


THE   PHARYNX, 


101 


The  shallow  and  widely  open  stumodajiim  6(jon  deepens  and  is  now  seen 
to  be  specially  bounded  by  certain  prominences  placed  above,  below,  and  at  the 
sides,  within  and  from  which  the  several  parts  of  the  face  arc  eventually  pro- 
duced (fif^s.  Ill,  111>).  These  pruminences  arc  {\\q  fronlo- nasal  which  projects 
over  the  stomoda^um,  and  is  formed  |)rimarily  by  tlie  i)rumiiience  of  the  fore- 
brain,  but  afterwards  acfjuires  a  considerable  thickeninir  of  mesoblast,  which  ex- 
tends into  it  from  the  basis  cranii ;  the  mandibular  or  f[rs^  j;yirfrnl  nrrh   wliirh 

Fig.  119. — PKotMLK  VIKW  UK  A  HIM.VN  KMURYO  OK 
AIU'IT  THKKK  WKKKS,  SHuWIXO  ALL  THE 
CKl'lIALIC    VISCKKAL    AKCHKS    AND    CLEFTS. 

wx,  maxillary  process  ;  mn,  niandiltular  arch  ; 
d.C,  duct  of  Ciivicr  ;  ji\  jugular  vein  ;  c.r,  cardinal 
vein  ;  r.v,  vitelline  vein  :  u.v,  umbilical  vein  ;  u.a, 
umbilical  artery  ;  all,  allantois  ;  jtl,  placentjil  attiich- 
ment  of  allantoic  stalk  ;  olf,  olfactory  depression  ; 
ot,  otic  vesicle. 

aftev_j2S§§ifl^— ^^^lil^J~I2-^yL^Jii^  &*^'^' 
gut,  takes   a   horizontal^  direction  on  its 

vcntftil  side,  and  m cc,t,^_lts  JteIlQw7jjQ_ tlie 
middle  line,  the  two  together  forming  the 
ventral  boundary  of  the  stpmodffilimT"a5id 
the  maxillarij  irracess^  which  grows  from 
the  base  of  the  mandibular  arch,  "and 
in'ojects  on  either  side  of  the  stomodfeum, 
filling  up  the  gap  between  the  fronto- 
nasal j2i"oce8S  and  the  maiidibular  arch, 
and  forming,  the  lateral  boundaiy. 

The  separation  of  the  stomodaeum  into  an 
upper  or  olfactory  and  respii-atoiy  part  and  a 
lower  permanent  buccal  ca\-ity.  together  with 
the  changes  which  occur  in  the  fronto-nasal 
and  maxillaiy  processes  to  produce  the  result,  has 
already  been  referred  to  in  describing  the  de- 
velopment of  the  nose  (pp.  9.5  to  97). 

Fliar3rnz. — The  remainder  of  the  ali- 
mentary canal  below  the  mouth  is  nearly 
simple,  at  first  consisting,  as  Jjefore  nien- 
tioned,_of  a  tuljular  portion  in  front-— the 
fore-gut  :  a  shorte_r^tubular  portion  behind 
— thelimcl-oiit  :  and  a  middle  part  which 
is    freely   open    to   the   yolk    (fig.  117). 

The"Tiirid-gut  remains  simple  throughout,  except  that  the  allantois  grows  out 
from  its  ventral  aspect.^  But  in  front  a  differentiation  soon~makes  its  appear- 
ance, the  j;ephaljjL-Part  bgiiiUiiag^,enJarged  to  fm-m  t,hp  phpiypv  while  almost 
immediately  behind  this  another  enlargement  forms  the  stomach.  The  lij:pi)lilast 
lining  tlir  ia\  ity  of  the  pharynx  grows  out  on  either  side  successively  at  foui"  distinct 
levels,  and  to  a  less  extent  flie  epiblast  dips  in  opposite  the  hypoblastic  outgrowths. 
In-this  way  eventualTy^four  deep  cleTts  Ijetween  the  pharynx  and  the  exterior  become 
formed  ;  these  are  Yvi()V,'\V^WQ  'ce£ha^^^  defis  (fi^.  110,  110,  in  external 

'  In  the  human  embryo  the  allantois  appears  to  be  formed  by  a  direct  continuation  of  the  lateral 
folds,  which  have  united  to  form  the  main  alimentary  tube,  while  the  part  of  the  tube  behind  the 
allantois  (bursa)  appears  as  a  blind  protrusion  (His). 


102  DEVELOPMENT    OF    THE    TONGUE. 

appearance  ;  f5gs.  117,  118,  seen  from  within). i  Between  them,  and  also  in  front  of 
the  first  cleft,  the  pharyngeal  wall  is  greatly  thickened  so  as  to  exhibit  the  appear- 
ance of  curved  bars  bounding  the  clefts  ;  these  bars  are  known  as  the  cephalic 
visceral  arcltes,  and  are  five  in  number,  viz.  :  the  Jir^stjii.Jmndthidar,  'in  front  of  the 
first'Visceral  cleft,  "b'etween  it  and  the  mouth  :  this  is  the  seat  of  formation  of  the 
lowerjaw  ;  the  second  or  Jiyoid  arch,  between  the  first  and  second  clefts  ;  the  third 
<yv~thyro-hiJoid  "arch,  between  tlie  second  and  third  clefts,  which  in  fishes  and 
amphibia  develops  gill-plates,  and  is  therefore  also  known  as  the  first  branchial 
arch  ;  \X\^  fourth  l^etween  the  third  and  fourth  clefts,  corresponding  with  the  second 
branchial  arch  of  fishes,  but  small  and  inconspicuous  in  man  and  mammals  ;  and  the 
p.fth  or  second  Iranchial  still  smaller  and  more  inconspicuous,  forming  the  posterior 
boundary  of  the  fourth  cleft,  and  hardly  recognizable  as  a  distinct  bar  in  man. 
After  the  fourth  week,  and  with  the  increasing  flexure  of  the  head,  the  arches  become 
somewhat  shifted  over  one  another,  so  that  the  fourth  arch  is  concealed  by  the  third, 
and  the  third  by  the  second. 

The  mandibular  arches  early  become  united  on  the  ventral  aspect  ;  from  the  fifth  week 
their  union  is  complete  (fig.  Ill,  7/;//)  and  in  man  shows  eventually  no  sign  of  a  median  groove. 
The  other  arches  do  not  at  first  reach  the  middle  line  (His),  the  space  between  their  central 
ends  being  occupied  by  the  heart  and  ijericardium  ;  as  these  shift  backwards  a  smooth  infra- 
mandibular  surface  is  left  externally. 

Development  of  the  tongue. — Within  the  pharynx,  the  second  and  third 
arches  of  the  two  sides  are  separated  by  a  foT-kecf  elevation  {furcuia)  with  a  median 

Fig.  120. — Posterior  aspect  of  the  visceral  arches  op  the  embryo 

SHOWN    IN    figs.    116,    117,    AS    SEEN    FROM    THE    INTERIOR    OF    THE 
PHARYNX.       (His.)       ^f 

The  first  or  mandibular  pair  of  arches  join  in  the  middle  line  ;  the 
second  arches  are  separated  by  a  rounded  j)rominence  (tuberculum 
impar).  Behind  (below)  this  is  the  forked  prominence  (furcula) 
bounding  a  median  groove  which  will  become  the  laryngeal  orifice.  In 
the  sections  of  each  of  the  first  two  arches,  the  included  artery  is  seen. 
The  Roman  numerals  are  opposite  the  corresponding  arches. 

groove,  in  front  of  which  is..a  I'ouaided.  tubercle  {t.  impar, 
His),  which  arTses,  in ,_ the;.  ,arLg.ular  space  l^etween  the 
first  and  seQpnd  arches  (fig.  120).  The' groove  around 
the  furcula  {sinus  arcuatiis.  His)  passes  laterally  into 
the  visceral  clefts.  The  second  and  third  arches  afterwards  unite  between  the 
furcula  and  tuberculum  impar  (fig.  121,  A).  Thus  united,  the  junction  forms  an 
X-shaped  mass. 

From  these  conjoined  ej.tiieniities  of  the  second  and  t]hjx.d  arches  on  either  side,  the 
root  of  the  tongue  grows  upwards  and  forwards  as  two  prominences,  which  diverge' 
in  a  V-shaped  manner  to  embrace  the  anterior  or  papillary  part  of  the  organ  which 
is  developed  from  the  tuberculum  impar  (fig.  121,  B).  At^tlje  angle  of  the  V  is  a 
deep  depression  {foramen  ccecum)  ;  this  leads  into  a  diverticulum,  which  forms  the 
median  rudiment  of"the"t"Hyroid  body.  When  the  parts  of  the  tongue  are  united, 
there  is  still  for  a  considerable  time  a  V'Sliaped  groove  marking  the  line  of  union 
(fig.  122),  and  even  in  the  adult  there  is  often  a  distinct  trace  of  this  groove  (sulcus 
terminalis,  His).  Parallel  to  this,  and  somewhat  in  front  of  it,  the  papilla  vallatse 
are  developed,  and  in  front  of  these  the  other  hngual  papilla  make  their  appear- 
ance (about  the  end  of  the  second  month). 

^  According  to  His,  who  is  confirmed  by  Born  and  by  Kolliker,  these  clefts  are  not  as  a  rule  developed 
into  complete  apertures  in  birds  or  mammals  ;  although  the  membi'anes  which  close  them  are  composed 
only  of  juxtaposed  epi-  and  hypoblast,  the  mesoblast  having  disappeared. 


1>K\  KLOI'MKN'I"    OK    TIIK    'l'()N<  I  UK. 


103 


'Vhv  fiirciila  p^ivcs  riso  to  llir  cpii^Icittis  in  front  (above)  tin-  aryepiirlottic  folds  on 
cither  sitk',  and  the  aryti'noid  cjirtdiiL^'es  beliind  (below) ;  the  median  jjroove  iu  it  loads 
to  the  entrance  of  the  larynx. 

Laterally  the  2ncl  arch  passes  into  and  forms  the  palato-trlossal  arch,  and  in  the  visceral 
cleft  behind  this  the  tonsil  develops  ;  hut  the  ;!rd  arch  docs  not  form  the  palato-pharyntrcal 
arch  ;  this  is  dovcluiicd  from  the  i)alatiuc  out',M-o\vtlis  of  the  maxillary  processes. 

The  visceral  arches  were  first  described  by  Hathkc,  in  isi'.'».  Tliey  are  often  dis- 
tinjjuished  as  the  pod -buccal  viscei'al  arches ;    certain  pai'ts  in  front  of  (above;  the  mouth 


Fig.  121. —Similar  vikw>  of  thk  samk  parts  in  older  embryos.     (His.)     A.  Y,  B.  'f 

T,  tuberculum  impr.r. 

beinpr  considered  by  some  morpholog'ists  to  represent  pirr-hnccal  arches.  The  visceral  clefts, 
lying  between  the  arches  are.  as  has  been  stated,  four  in  number.  The  first  is  often  known  as 
the   Injomandiljuhir  cleft  :    it   is  this   one   which  is   concerned  with   the   formation   of   the 


Fig.  122. — Similar  view  in  a  considerably  older 

EMBRYO,    BUT    LESS    MAGNIFIED.        (Hi.S. ) 

Eustachian  tube  and  middle  ear  as  already  de- 
scribed. The  three  remaining  clefts,  which  repre- 
sent gill-slits  of  fishes  and  amiihibia.  appear,  from 
the  results  of  recent  observations,  to  be  closed  in 
amniotic  vertebrates  at  all  periods  of  foetal  life 
(see  note  on  previous  page).  In  some  fishes  the 
branchial  arches  and  clefts  are  more  numerous 
than  in  other  vertebrates,  and  in  a  few  the  hyoid 
arch  also  develops  a  gill. 

Through  each  of  the  visceral  arches  an  arterial 
arch  derived  from  the  aoi-tic  bulb  passes  from 
front    to    back    reuniting    dorsally    in    front    of 

the  notochord  to  form  the  aorta.  In  branchiate  vertebrates,  branches  of  these  vessels  are 
distributed  to  the  gills.  Cartilaginous  bars  pass,  iu  most  vertebrates,  from  the  base  of  the 
skull  into  each  visceral  arch,  and  ossification  occui-ring  in  or  around  them,  form  definite  parts 
of  the  skeleton  as  will  be  afterwards  described.  In  man  and  mammals  these  cartilaginous 
Vjars  are  only  found  in  the  first  three  visceral  arches,  unless  the  thyroid  cartilage  is  to  be 
regarded  as  representing  the  anterior  (ventral)  ends  of  the  bar  of  the  fth  arch  (Callender). 
The  fourth  and  fifth  visceral  arches  may  be  considered  as  belonging  to  the  neck  rather 
than  to  the  head,  and  the  congenital  fissures  of  the  neck  which  sometimes  occur  as  a  mal- 
formation, and  which  usually  open  externally  far  down  in  the  cervical  region,  have  been 
regarded  as  due  to  persistence  of  one  or  more  of  the  branchial  clefts,  shifted  in  position  by  the 
cervical  elongation  which  takes  place  in  later  emljr3'onic  life. 

Olsophagus,  stomach,  and  intestines. — Immediately  behind  the  pharynx,  the 
fore-gut  contracts  again  to  form  the  oesophagus,  which,  in  the  early  emljryo,  corres- 
ponding with  the  imperfect  development  of  the  neck,  is  very  short  (figs.  123, 125,  A) 


104 


i 

(ESOPHAGUS.    STOMACH,    AND    INTESTINES. 


and  gradually  widens  out  into  the  dilatation  which  represents  the  stomach. ^  This 
organ,  which  is  at  first  nearly  straight  (fig.  125,  A,  3Ig),  soon  begins  to  show  the  con- 
Fig.  123. — Sketch  of  a  longitudinal  section  through 

THE  alimentary  CANAL  OF  A  HUMAN  EMBRYO, 
SOON  AFTER  THE  DISAPPEARANCE  OF  THE  PRIMITIVE 
VELUM.      (His.)      *f 

The  alimentary  canal  is  shaded  throughout ;  U.  K, 
section  of  mandibular  arch  ;  R.  T,  hypophysis  ;  behind 
it  the  I'emains  of  the  pharyngeal  septum  ;  Lrj,  com- 
mencing lung,  the  future  orifice  of  the  lai'jnx  being 
opposite  K  ;  Mg,  stomach  ;  Lb,  liver ;  Nb,  yolk  stalk  ; 
W,  Wolffian  duct  ;  B,  blind  portion  of  hind  gut ; 
all,  allantois. 

vexity  of  the  greater  curvature  on  the  side  next 
the  vertebral  column,  and  the  concavity  of  the 
lesser  curvature  on  the  opposite  border  (fig, 
125,  B,  Mg),  while  the  pyloric  end  becomes 
tilted  away  from  the  vertebral  column,  pro- 
ducing the  duodenal  loop  (fig.  125,  C,  D_). 
Finally  the  organ  becomes  turned  over  on 
W'hat  was  previously  its  right  side,  which  now^ 
becomes  the  posterior  surface,  and  the  pyloric 
extreuiity  being  also  tilted  over,  the  duodenal 
loop  is  thus  thrown  over  to  the  right  side  of  the 
abdomen  (fig.  126).  The  small  intestine  is  also 
at  first  quite  short  and  straight,  with  a  wide 
aperture  to  the  yolk-sac  (fig.  125,  A,  Nh), 
but  gradually  lengthens  as  the  communication 
with  the  yolk-sac  becomes  more  contracted, 
and  (besides  the  loop  formed  by  the  tilting  of 
the  pylorus)  develops  a  long  V^sbaped  loop 
opposite  the  attachment  of  the  vitelline  duct 
(fig.  125,  C,  t),  and  fig.  127). 

The  loop  of  intestine  to  which  the  vitelline  duct 
is  attached  passes,  for  a  time,  into  the  umbilical 
cord,  close  to  its  attachment,  enclosed  in  a  protrusion  of  the  peritoneal  cavity  (fig-.  124).     It 
occasionally  remains  in  this  situation  until  late  in  foetal  life. 

Fig.  12'1. — Sketch   of   the   human    embryo    of    the 

TENTH    WEEK,    SHOWING    THE    COIL    OF    INTESTINE    IN 

THE  UMBILICAL  CORD.     (Allen  Thomson.) 

The  amnion  and  villous  chorion  have  been  opened  and, 
the  embryo  drawn  aside  from  them  ;  'v,  umbilical  vesicle, 
connected  with  the  coil  of  intestine,  i,  by  a  small,  almost 
linear  tube.  The  figure  at  the  side  represents  the  first 
part  of  the  umbilical  cord  magnified  ;  i,  coil  of  intes- 
tine ;  vi,  vitelline-intestinal  duct,  alongside  of  which 
are  seen  omphaJo-mesenteric  blood-vessels. 

The  mesentery  is  developed  by  a  thinning 
out  and  extension  of  the  mesoblastic  tissue 
W'hich  lies  between  the  intestine  and  the 
vertebral  column.  It  forms  a  continuous  membrane  along  the  wdiole  length  of  the 
alimentary  canal  from  the  stomach  to  the  rectum,  although  the  part  attached  to  the 

1  It  has  been  shown  (Balfour,  Meuron)  in  most  vertebrates— mammals  excepted— that  at  a  certain 
period  of  development  the  lumen  of  the  oesophagus  becomes  for  a  time  completely  obliterated  at  its  upper 
extremity. 


(KSorilAdUS,    STOMACH,    AND    I.XTESTINRS. 


105 


Fig.  125. — Profile  sketches  of  successive  stages  in  the  development  of  the  alimextary  canal 

IN  the  human  embryo.     (His.) 

Ch,  notochord  :  Sd  (in  B),  median  rudiment  of  thyroid  ;  P,  pancreas  ;  Lhf),  bile  duct  ;  Ds,  vitelline 
duct  ;  Zfj  (in  C  and  1)),  tongue  ;  N,  permanent  kidney  ;  d  (in  D),  cloaca  ;  An,  anus  in  coui-se  of  form- 
ation ;  .SV/,  sexual  prominence  ;  St,  tail  ;  Cc,  cceciim  coli  ;  Tr,  ti-acbea  ;  A',  larynx.  The  other  letter- 
ing as  in  tig.  123. 


106 


CESOPHAGUS,    STOxMACH,    AND    INTESTINES. 


stomach  is  known  as  mcsogastriuni  and  the  parts  attached  to  the  future  colon  and- 
rectum  are  termed  respectively  mesocolon  and  mesorectum.  The  stomach  begins  to 
assume  its  characteristic  shape  while  still  lying  with  its  longitudinal  axis  in  the 

Fig.  126. — Front  view  of  alimextary  cakal,  rather  LEf^s  adyaxced  in 

DEVELOPMENT  THAN  THAT  SHOWN  IN  PIG.  12.5,  D.   (His.)  V) 

The  pharynx  and  upper  part  of  cesophagus,  and  termination  of  the  large 
intestine  are  not  represented.     Lettering  as  in  fig.  125. 

median  plane  of  the  body  ;  it  is  then  seen  that  the  mesogastrium 
passes  to  its  greater  curvature  (fig.  127),  which,  therefore,  is 
that  corresponding  to  the  mesenteric  border  of  the  intestine. 
And  as  the  pyloric  extremity  of  the  stomach  and  lesser  curva- 
ture are  tilted  forwards  and  upwards,  and  at  the  same  time 
the  whole  organ  turns  over  on  its  right  side,  the  mesogastrium 
becomes  proportionally  lengthened  to  permit  of  this  change  of 
position,  and  the  right  surface  of  the  stomach  (now  posterior) 
rests  against  the  anterior  surface  of  what  was  previously 
the  right  side  of  the  mesogastrium,  the  mesogastrium  thus 
coming  to  form  the  posterior  boundary  of  the  omental  sac  (fig. 
128).  From  near  its  attachment  to  the  stomach  a  free  fold 
subsequently  grows  over  the  intestines,  and  becomes  the  great 
omentum. 
The  gastro-liepatic  omentum  is  formed  by  the  gradual  thinning  of  a  mass  of 
mesoblastic  tissue  which  from  the  first  connects  the  ventral  wall  of  the  stomach  with 
the  anterior  wall  of   the  abdomen,  and  mthin  which  the  hypoblastic  outgrowth 


ZZy. 


Fig.   127.— DlAGEA3I   OF   THE    MESENTERY,    STOMACH    AND    INTESTINE    OF    A   HUJIAN    EMBRYO    OF    SIX 

WEEKS.     (Toldt. ) 
&t,  stomachL ;    y.c,   greater   curvature;    l.c,   smaller   curvature;   mg,  mesogastrium ;_  sj:!?,   spleen; 
p,  pancreas  ;  c,  CEecum  ;  r,  rectum  ;  me,  mesentery ;  ao,  aorta  ;  cl,  cceliac  axis ;  s.me&.a,  i.mes.a,  supe- 
rior and  inferior  mesenteric  arteries. 

Fio-.  128. — Diagram  of  a  section  across  the  abdomen  of  a  human  embryo  of  the  third 

MONTH.      (Toldt.) 

I,  I,  liver  ;  k,  kidneys  ;  g.o,  gi-eat  omentum  ;  f/.o',  omental  sac  :  s.o,  small  omentum,     (The  dotted 
lino  has  not  lieen  carried  quite  far  enough.)     The  other  letters  as  in  fig.  127, 

which  forms  the  liver  becomes  developed.  The  part  of  this  mesoblastic  connexion 
which  lies  between  the  liver  and  stomach  becomes  the  gastro- hepatic  or  lesser 
omentum,  and  its  free  border  which  was  at  first  directed  downwards  (caudalwards) 
becomes  with  the  descent  of  the  stomach  directed  anteriorly  (ventrally),  and  eventu- 
ally with  the  turning  of  that  organ  laterally  it  also  is  directed  towards  the  right,  and 
thus  comes  to  form  the  anterior  boundary  of  the  entrance  into  the  omental  sac. 


(KSol'llACfS,    STOMACH,    AND    IN'IKSTI  N  KS. 


107 


The  large  intestine  is  not  at  first  marked  oil'  from  the  small  \>y  any  dill'erenee  in 
calibre.  Its  commencement  is  distin<,'uishal)le  about  the  sixth  week  in  the  human 
emltryo  by  the  appearance  of  the  caecum,  whieli  jrradually  j,fro\vs out  (Hgs.  \2'>,  D,  and 
lL^7),  terming  at  first  a  lateral  pn^trusion  of  uniform  calibn-,  but  sul)SL(juently  re- 
maining narrow  at  its  blind  extremity  to  form  ihe  vermiform  appendix,  while  the 
i-emainder  of  the  caecum  and  the  colon  increase  in  size.  This  protrusion  occurs  on 
the  [J"'^'>"P^''^  '""P  '"I'jtjve  deserilx'd,  and  a  little  beyond  the  attachment  of  the 
vitelline  duet. 

AVith  the  iiicreasini:-  length  of  the  gut  it  becomes  thrown  into  coils,  and  the 
earliest  and  most  important  of  these  is  that  by  which  the  limb  of  the  [J -''J^'ip*-''^  ^^^V 


d'l^^^^^^^^ 


dicif^f^r,^^ 


'/lancreas 


■cdenun 


testine. 


Fig.  129. — Diagrams  illcstkatixg  the  i^eveloioiext  of  the  great  ojie>tl-3I.     (0.  Hertwig.) 

A,  earlier  stage. 

B.  later  stage. 

St,  stomach  ;  s.o,  small  omentum  :  s'.o',  omental  sac  :  o',  mcsogastrium,  springing  from  the 
posterior  wall  of  the  abdomen,  near  which  in  A  it  encloses  the  pancreas  ;  o-,  attachment  of  mesogastrium 
to  greater  ciu-vature  cf  stomach  ;  (/\  fold  of  mesogastrium  or  gi-eat  omentum  growing  over  coils 
of  small  intestine;  me,  mesentery;  m.c,  transverse  mesocolon:  o"*  (in  B),  dotted  line  showing  the 
situation  of  that  lamella  of  the  mesogastrium  which  at  first  assisted  in  enclosing  the  pancreas  but 
which  has  now  disappeared.  The  next  part  of  this  lamella  has  coalesced  with  the  adjacent  lamella  of 
the  transverse  mesocolon,  and  has  also  disajJijeared.     The  coalescence  is  indicated  by  the  black  line. 


with  which  the  large  intestine  is  continuous  turns  over  on  to  the  right  side  of  the 
peritoneal  cayity,  and  thus  throws  the  colon  in  an  archlike  disposition  across  the 
commencement  of  the  small  intestine,  and  parallel  with  the  longitudinal  axis  of  the 
stomach  (commencement  shown  in  fig.  12G).  Within  this  arch  of  the  large  intestine 
the  coils  of  the  jejuniun  and  ileum  become  disposed  as  the  intestine  lengthens. 
Their  mesentery  spreads  out  at  its  intestinal  attachment  so  as  to  adapt  itself  to  the 
increasing  length  of  the  gut,  Avhile  its  Tcrtebral  attachment,  relatively  much  shorter, 
loses  to  a  great  extent  its  primitive  disposition,  and  acquires  oblique  and  transverse 
lines  of  attachment ;  this  is  notably  the  case  Avith  the  transverse  mesocolon. 

Althougrh.  the  mesentery  in  most  parts  increases  in  lengrh  and  expansion  with  the  further- 
growth  of  the  intestine,  the  contrary  is  the  case  -nith  the  mesenterj'  of  the  duodenum,  and  of 
the  ascending-  and  descendinfr  colon.  All  these  parts  possess  at  fiist  a  complete  mesentery  like 
the  rest  of  the  intestine,  but  that  of  the  duodenum  disappears  entirel}-,  so  that  this  part  of  the 
intestine  becomes  fixed  to  the  posterior  wall  of  the  abdomen,  and  the  same  process  takes  place 
to  a  lesser  and  variable  extent  with  the  ascending  and  descending  mesocolon.  Since  the  trans- 
verse colon  lies  across  the  abdomen  immediately  below  the  stomach,  it  and  its  mesentery,  trans- 
versely disposed,  also  lie  immediately  below  and  behind  the  mesogastrium  (now  folded  into  the 
great  omentum).    The  two  membranes  come  in  fact  into  close  contact,  and  eventually  com- 


108  FORMATION    OF    THE    ANUS. 

pletely  adhere  (4th  month  and  onwards)  :  and  this  causes  the  panci'eas  to  appear  to  lie  alto- 
gether behind  the  peritoneal  cavity,  in  place  of  being-  situated  between  the  two  layers  of  the 
mesogastrium  as  is  at  first  the  case  in  the  human  embryo,  and  as  is  frequently  found  in  other 
mammals  during-  life. 

The  free  or  iioating  part  of  the  great  omentum  is  formed  by  an  extension  of  that  part  of 
the  omental  fold  which  turns  upwards  towards  the  greater  curvature  of  the  stomach  from 
the  surface  of  the  transverse  colon.  The  fold  is  at  first  clearly  double,  and  in  some  animals 
remains  so,  but  in  man  its  two  layers  coalesce  a  little  while  after  birth,  and  after  a  year  or 
two  can  no  longer  be  separated.  It  extends  gradually,  first  over  the  transverse  colon  (third 
month),  later  over  the  coils  of  the  small  intestine. 

The  spleen  becomes  formed  within  the  substance  of  the  mesogastrium  (fig. 
127,s2jI).  It  is  developed  \Yholly  from  mesoblast,  and  in  close  connection  with  the 
pancreas.  It  appears  during  the  second  month  in  the  human  embiyo,  and  grows 
slowly  during  faial  life,  the  Malpighian  corpuscles  being  the  last  parts  to  appear. 

Formation  of  the  anus. — The  anal  invagination  of  the  epiblast,  which  eventu- 
ally by  absorption  of  the  septum  between  it  and  the  hypoblast  of  the  hind-gut  opens 
into  the  alimentary  tube,  is  termed  the  procfodceum.  The  junction  with  the  hind- 
gut  occurs  at  a  little  distance  from  the  posterior  extremity,  so  that  there  is  a  blindly 
terminating  post-anal  or  subcaudal  portion  of  the  gut  beyond  the  junction  with  the 
profitodgeum  ;  this,  however,  shrinks  and  disappears  even  before  the  absorption 
of  the  sejDtum.  This  part  of  the  hind-gut  represents  a  cloaca,  since  it  receives 
through  the  allantois  the  ducts  of  the  urinary  and  genital  organs.  The  sepa- 
ration of  the  permanent  anus  from  the  urogenital  orifice,  which  occurs  in  all 
mammals  above  monotremes,  is  the  result  of  a  later  process  of  development  (see 
p.  127). 

In  mammals  the  actual  amount  of  proctodasal  invagination  is  very  small.  The 
septum  between  the  hind-gut  and  the  exterior  (anal  membrane)  is  throughout  formed 


Fig.    130. LONGITTJDIKAL    SECTION    THROUGH  THE    POSTEEIOR    END    OF    A   SHEEp's    EMBRYO,    SHOWING    THE 

ANAL  MEMBRANE.     (Bonnet.) 

cp,  epiblast ;    hi/,  bypoblast ;    mes,  mesoblast ;    Ji.g,  hindgut ;    a7n,  amnion  ;    an,  anal  membrane  ; 
p.s,  prindtive  streak  ;  all,  allantois-rudiment. 

by  two  epithelial  layers  only,  viz.,  hypoblast  and  epiblast,  Avhich  here  are  in  contact 
with  one  another  without  the  intervention  of  mesoblast  (fig.  130,  an).  This  con- 
dition of  juxtaposition  of  the  two  layers  is  in  fact  directly  derived  fr'om  the  union 
of  the  two  layers  which  occurs  at  the  primitive  streak  and  groove,  and  if  the  latter 
be  looked  upon  as  representing  the  blastopore,  the  anus  may  in  a  sense  be  considered 
to  be  formed  from  a  part  of  that  aperture.  In  some  lower  vertebrates  the  anus  has 
been  shown  to  be  directly  produced  from  the  blastopore. 


THE    LUNGS. 


109 


FORMATION   OF   THE    GliANDS   OF   THE   ALIMENTARY   CANAL. 

Under  this  licatl  may  lie  iiicliidiMl  not,  only  tliose  orj^iuis  which  an-  ordinarily  so 
termed,  but  also  the  lungs,  and  the  ihyiuus  and  thyroid  Ixtdics,  since  the  early 
developiiK-nt  of  these  three  orj^ans  resenibles  that  of  the  true  scei'eting  <,dands. 

All  the  organs  above  enumerated  are  formed  as  epithelial  involutions,  either  solid 
at  fii-st  and  afterwards  becoming  hollowed  out,  or  hollow  from  the  first.  As  these 
epithelial  buds  grow  into  the  mesoblast,  they  may  either  bifurcate  or  give  off  lateral 
branches,  and  in  this  manner  all  the  ramifications  of  the  ducts  of  the  compound 
racemose  glands  are  produced.  The  blind  extremities  generally  end  eventually  in 
enlarged  tubular  or  saccular  dilatations.  All  the  epithelium  of  the  gland-saccules 
and  ducts  is  derived  from  the  original  epithelial  sprout,  while  the  basement 
membranes  and  connective  tissue  and  blood-vessels  of  the  gland  are  derived  from  the 
surrounding  mesoblast.  The  salivary  glands  and  most  other  glands  of  the  mouth, 
and  part  of  the  hypophysis,  which  must  also  be  reckoned  as  a  glandular  development, 
are  formed  in  this  way  by  involution  of  the  buccal  or  stomodseal  epiblast ;  while  the 
lungs,  liver,  pancreas,  thyroid,  thymus,  and  all  the  small  glands  of  the  rest  of  the 
alimentary  canal  are  formed  of  involutions  of  the  hypoblast.  The  development  of 
the  teeth,  which  also  first  make  their  appearance  as  involutions  of  stomodajalepi blast 
(enamel  germs),  will  be  described  after  their  structure  has  been  dealt  with  (in  the 
part  of  this  work  which  is  devoted  to  Splanchnology). 

The  lungs.— The  lungs  begin  to  develope  from  the  ventral  part  of  the  pharynx 
at  its  junction  with  the  cesophagus,  in  the  beginning  of  the  third  day  in  the  chick. 


Fig.    131. LCNG    RrDIMF.NTS    OF    HUMAN    EMBRYO    OF    ABOUT    4    WEEKS,   SHOAVING    THE  BUD-LIKE 

EXLARGEMENTS    WHICH    REPRESENT    THE    LOBES    OP    THE    FUTURE    LUNGS.       (His. ) 

Three  buds  are  seen  on  the  right  side,  two  on  the  left. 
Fig.  132. — Lungs  of  a  human  embryo  muke  akvanced  in  development.     (His.) 


and  in  the  human  embryo  at  a  coiTespondingly  early  period  (fig.  123,  Lg).  The 
lung  rudiment  is  at  first  single  and  median,  and  takes  the  form  of  an  elongated 
vertical  diverticulum  of  the  fore-gut,  communicating  freely  with  that  tube,  and  of 
course  lined  by  hypoblast.  Soon  the  diverticulum  sprouts  out  at  its  lower  extremity 
in  the  form  of  two  tubes  which  grow  downwards  on  either  side  behind  and  on  either 
side  of  the  heart,  into  a  mass  of  mesoblastic  tissue,  which  keeps  pace  in  its  growth 
with  the  hypoblast ic  lung  rudiment,  and  from  which  the  connective  tissues  of  the 
future  lung  become  ultimately  developed.  The  extremities  of  the  tubes  in  question 
are  early  seen  to  be  dilated  and  lobulated  (fig.  131),  three  lobules  being  present  on 
the  right  tiibe,  and  two  on  the  left,  the  division  of  the  lungs  into  their  lobes  being 
thus  early  indicated. 

The  further  outgro^vth  of  the  lobulations  produces  the  rudiments  of  the  principal  branches 
of  the  bronchi,  one  for  each  future  pulmonai-y  lobe,  and  each  of  these  branches  then  gradually 


110 


THE    THYEOID    BODY. 


Fi 


133. — LtJNGS    OF    A    HUMAN    EMBRYO 
STILL    MORE    ADVANCED.       (His. ) 


progresses  in  growth,  gciving  off  as  it  proceeds  lateral  diverticula,  which  form  the  secondaiy 
bronchi,  and  these  again  giving  off  others  until  the  whole  complicated  bronchial  ramification 
is  eventually  produced.  Like  the  first  sprouts  from  the  median  diverticulum,  all  the  secondary 
and  other  sprouts  are  dilated  at  their  termination,  and  have  a  lobulated  aspect  (fig.  125,  Lrj  ; 

figs.  131,  132,  133.  This  is  due  to  the  fact  that  they 
are  undergoing  a  further  division  or  sprouting.  This 
process  goes  on  until  the  sixth  month  of  intrauterine 
life,  by  which  time  all  the  dilated  ends  of  the  growing 
and  sprouting  tubes  have  reached  the  surface  of  the 
lung.  These  dilated  extremities  which  now  appear 
grouped  together,  and  apparently  springing  several 
from  a  common  tube,  form  the  infundibula,  but  their 
walls  are  not  at  first  beset  with  air-cells.  The  forma- 
tion of  these  takes  place  when  the  bronchial  ramifica- 
tion is  completed  (sixth  month,  Kolliker),  as  small, 
closely-set,  pouch-like  protrusions  of  the  walls  of  the 
infundibula,  and  of  the  terminal  bronchial  tubes. 

The  trachea  and  larynx  are  formed  by  a 
separation  from  the  oesophagus  of  the  original 
median  diverticulum,  from  the  lower  angles  of 
which  the  bronchial  rudiments  have  sprung,  the 
separation  commencing  below,  and  leaving  a 
relatively  small  connection  between  the  two 
tubes  above  :  this  connection  is  the  rudimentary  glottis.  As  development  advances, 
both  the  tracheo-laryngeal  and  the  oesophageal  tubes  lengthen,  the  latter  relatively 
more  than  the  former,  so  that  the  lung  rudiments  no  longer  lie,  as  was  the  case  at 
first,  in  front  of  and  on  either  side  of  the  stomach,  but  extend  downwards  somewhat 
short  of  thai  organ  (fig.  125),  separated  from  one  another  by  the  oesophagus  behind, 
and  the  heart  and  pericardium  in  front.  As  they  thus  grow  backwards  with  the 
lengthening  of  the  trachea,  the  lung  rudiments  project  into  the  anterior  part  of  the 
body-cavity  or  coelom  (dorsal  portion),  and  receive  a  covering  fi-om  its  lining  mem- 
brane, at  first  only  below  and  on  the  external  surface,  but  subsequently  on  the 
internal  aspect,  so  as  to  separate  them  from  the  oesophagus.  The  portions  of  the 
body-cavity  into  Avhich  the  lungs  project  become  shut  off  from  the  remainder  on 
the  formation  of  the  diaphragm  and  pericardium,  and  form  the  pleura. 

The  pulmonary  blood-vessels  are  comparatively  late  in  being  developed,  the 
arteries  penetrating  into  the  lung  tissue  only  on  the  twelfth  day  in  the  chick. 

The  thyroid  body  is  developed  partly  as  a  median  diverticulum  of  the  pharyn- 
geal hypoblast  opposite  the  ventral  ends  of  the  second  visceral  arches  (fig,  ]  25,  B,  Sd); 
partly  as  a  (bilateral)  diverticulum  of  the  posterior  wail  of  the  fourth  visceral  cleft. 
The  median  diverticulum  in  most  animals  early  becomes  separated  from  the 
pharyngeal  hypoblast,  and  is  thus  converted  into  an  island  of  epithelium  imbedded 
in  mesoblast.  In  the  human  embryo,  as  His  has  shown  (fig.  134,  A,  thr),  it  remains 
for  some  time  in  the  form  of  a  hollow  bifid  vesicle,  which  is  connected  with  fhe  upper 
surface  of  the  tongue  by  a  small  duct  {ductus  thyreogloss^is,  d) ;  subsequently,  however, 
the  vesicle  becomes  solid,  and  the  duct  is  obliterated  and  disappears,  with  the 
exception  of  a  small  portion  near  the  orifice,  which  becomes  converted  into  the 
foramen  ccpxum  of  Morgagni,  fx. 


Occasionally  even  in  the  adult  a  comparatively  long  duct  is  found,  leading  downwards 
and  backwards  from  the  foramen  csecum.  This,  which  has  been  termed  the  ductus  linr/imlis, 
is  the  remains  of  the  original  thyrolmgual  duct  connecting  the  median  part  of  the  thj^roid 
with  the  tongue.  It  may  further  happen  that  the  lower  part  of  this  connection  also  remains 
in  the  shape  of  a  tubular  prolongation  of  the  median  portion  of  the  thyroid  towards  the  root 
of  the  tongue  Qdnctus  thnjoideus  ;  when  well  developed  this  forms  the  2)yram'id^.  The  so- 
called  accessory  thyroid  bodies  (supra-hyoid,  prEehyoid  glands,  &c.)  which  are  occasionally 
found   near   the   hyoid   bone,  are  also   referable   to   the   thyrolingual   duct  (His,  Anatomie 


THK    THYMUS. 


HI 


menschlichpv  Emhrvonen,  iii.,   p.    inl,  wlioro   roferonco   to  the  literature  of    these  acccpsory 
thyroi<ls  may  be  loiind). 

The  hihitcval  diverticulii,  which  assist  in  tlie  formation  of  the  thyroid  body,  spring 
from  the  fourth  visceral  cleft  (Born)  (Hi;-.  1;!."),  ///;•').  They  have  at  first  the  a])j)earance 
of  simi)le  saccuhir  j,dands  partially  encircling  the  developing  larynx  (fig.  l;M,  ////'). 
In  front  of  this  they  come  into  connection  with  the  median  rudiment,  and  eventually 
hlend  with  it.  Tjikc  thab  rudiment,  they  become  entirely  se))arated  from  the  hypo- 
blastic  surface  from  which  they  have  taken  origin,  their  cavity  disappears,  and  they 


Fig.  134. — Sketches  snowixo  the  condition  op  the  thyroid  and  thymcs  gland.s  in  a  human 

EMBRYO    or    ABOUT    FIVE    WEEKS.       (Hls.) 

A,  profile  sketch  fiom  the  left  side. 

B,  frontal  sketch  from  behind. 

t,  tongue  ;  fc,  foramen  ctecum  ;  d,  ductus  thyreoglo.ssus  ;  ep,  epiglottis  ;  opposite  I,  larj'nx  ;  tr, 
trachea  ;  ce,  oesophagus  ;  thr,  median  rudiment  of  thyroid  ;  thr',  lateral  rudiment  of  thyroid  ;  thm, 
developing  thymus,  seen  on  the  left  side  of  B  to  be  connected  with  a  visceral  cleft ;  ao  (in  B),  ascending 
aorta  ;  ao',  descending  aorta  ;  c,  cai'otid. 

are  converted  into  ramifying  and  anastomosing  cell-cyhnders,  between  which  vascular 
connective  tissue  becomes  developed.  The  cell-cylinders  subsequently  become  hol- 
lowed out,  and  finally  are  subdivided  by  growth  of  the  connective  tisstie  into  small 
vesicles,  which  gradually  become  larger  from  accumulation  of  colloid  in  their  interior. 

In  most  Vertebrates,  the  lateral  and  median  parts  of  the  thyroid  remain  distinct  ;  the 
former  are  the  organs  known  as  snpra-jjcrirardial  boflir.t  in  elasmobranchs,  and  as  arrrxsory 
tlnjrokls  in  other  animals.  Only  in  mammals  do  they  become  united  into  one  organ  as 
in  man. 

The  thynms  is  also  developed  as  a  growth  of  the  epithelium  (hypoblast)  of  some 
of  the  visceral  clefts  ;  in  birds  from  the  third  and  fourth  (fig.  13-"),  fh>/i)i),  in  reptiles 
from  the  second,  third,  and  fourth,  and  in  lower  Vertebrates  from  several  clefts  (de 

Fig.   13.5. — Diagram    sho'svinc;   the    visceral   clefts    from  which   the 

THYMUS    and     lateral     PARTS     OF     THE     THYROID    ARE    DEVELOPED    IN 

THE  chick,     (de  Jleurori. ) 

1,  2,  3,  4,  indicate  the  corresfionding  visceral  clefts  ;  thyni,  rudiments  of 
thymus  ;  thr,  median  rudiment  of  thyroid  ;  thr',  lateral  rudiments  of 
thyroid. 

IMeuron).     In  mammals  the  thymus  appears  as  a  (bilateral) 

tubular  prolongation   backwards   of  the   extremity  of   the 

third  visceral   cleft  (Kolliker),  receiving,  according  to  de 

IMeuron,  an  accession  from  the  hypoblast  of  the  fourth  cleft, 

as  in  birds.     The  tube,  which  has  a  narrow  lumen,  and 

comparatively  thick  epithelial  lining,  is  surrounded  by  vascular  connective  tissue, 

within  which  numerous  lymphoid  cells  become  accumulated,  and  grows  downwards 

along  the  side  and  in  front  of  the  trachea,  where,  in  mammals,  it  generally  unites 


112  THE    LIVER. 

with  its  fellow  to  form  a  median  organ.  Its  lower  end  then  gives  off  solid,  bud-like 
excrescences,  and  lateral  buds  come  off  again  from  these,  so  that  this  part  of  the 
organ  acquires  a  ramified,  lobulated  appearance  hke  an  acinous  gland.  The  acini  are, 
however,  solid,  and  remain  so,  although  the  upper  end  of  the  tube  still  has  a  narrow 
lumen. 

The  lymphoid  cells  next  invade  the  epithelium,  growing  into  every  part  of  the 
tubular  gland,  and  converting  it  into  a  mass  of  adenoid  tissue.  In  this  process  the 
epithelium  becomes  broken  up  into  small  isolated  portions,  some  of  which  remain  in 
the  medullary  portion  of  the  lobules  as  the  epithelial  nests  which  are  seen  in  sections 
of  the  fully  developed  organ,  and  are  known  as  the  concentric  corpuscles  of  Hassall. 

The  liver. — This  organ  arises  in  the  form  of  two  diverticula  of  hypoblast,  which 
grow  from  the  ventral  wall  of  the  duodenum  immediately  beyond  the  stomach  (figs. 
117,  118, 1, 123,  Lh).  They  extend  into  a  mass  of  mesoblastic  tissue  which  connects 
the  stomach  and  duodenum  with  the  anterior  wall  of  the  abdomen,  and  which  (with 
the  mesentery,  with  which  it  is  continuous  round  the  gut)  separates  the  body-cavity 
here  into  a  right  and  left  half.  In  this  tissue  is  the  omphalomeseraic  or  vitelline 
vein  (and  later  the  umbilical  vein)  proceeding  on  either  side  to  the  sinus  venosus, 
and  the  liver  diverticula  grow  into  the  mesoblast  above  and  in  front  of  these  veins. 
Here  they  ramify,  giving  off  solid,  buds  of  cells  which  groAV  into  columns  or  cylinders, 
and  these  again  give  off  lateral  diverticula  of  the  same  kind.  So  far  the  development 
of  the  liver  resembles  that  of  a  compound  tubular  or  acino-tubular  gland,  except  that 
the  ramifications  of  the  original  gland  diverticula  are  from  the  first  solid  instead  of 
hollow.  But  soon  an  important  difference  appears  in  the  fact  that  the  cylinders 
unite  and  anastomose  with  one  another  everywhere  to  form  a  close  network,  and 
from  the  cords  of  this  network  solid  sprouts  are  again  constantly  being  given  off  to 
form  fresh  cylinders,  thus  producing  a  yet  closer  and  more  intricate  network.  In 
the  meantime,  capillary  blood-vessels  are  formed  in  the  mesoblastic  tissue  in  which 
this  formation  of  cell-cylinders  of  hypoblast  is  going  on,  and  these  vessels,  which 
form  a  network  interlocking  with  that  of  the  anastomosing  cell-cylinders,  become 
connected  with  branches  of  the  vitelline  vein  on  the  one  hand  {vence  adveJientes), 
and  on  the  other  with  veins  (vence,  revehcntes)  which  pass  towards  the  sinus  venosus, 
and  eventually  are  found  opening  as  the  hepatic  veins  into  the  inferior  vena  cava. 

The  two  original  hollow  diverticula  are  the  rudiments  of  the  right  and  left  hepatic  ducts. 
The  common  bile  duct  is  formed  later  by  a  protrusion  of  that  part  of  the  duodenal  wall  with 
which  the  original  diverticula  are  connected.  This  protrusion  also  eventually  receives  the 
duct  of  the  pancreas,  which  becomes  shifted  towards  it.  As  the  common  bile  duct  lengthens, 
the  liver  becomes  separated  from  the  duodenum,  with  which  it  was  at  first  in  close  connection. 
The  portal  and  interlobular  bile  ducts  are  formed  by  the  hollowing  out  of  some  of  the 
anastomosing  cell-cylinders,  so  that  a  lumen  is  produced  within  them  surrounded  by  hepatic 
cells,  which  lose  their  original  polyhedral  character,  and  become  changed  into  the  columnar 
epithelium  of  the  ducts,  the  anastomoses  between  the  ceU-cylinders  here  disappearing. 
The  remaining  cylinders  form  the  secreting  substance  of  the  liver.  The  biliary  canaliculi 
appear  as  minute  passages  between  the  cells,  and  come  into  continuity  with  the  bile  ducts. 
With  a  further  development  of  the  connective  tissue  of  the  organ,  the  glandular  substance  of 
the  liver,  which  was  at  fii'st  continuous  throughout,  becomes  separated  into  lobules,  and  the 
network  of  cell-cylinders  tends  with  multiplication  of  their  cells  to  become  fused  into  a  con- 
tinuous mass  within  each  lobule,  the  bile  canaliculi  forming  by  numerous  lateral  junctions  and 
anastomoses  a  close  network  of  intercellular  passages  within  the  lobule. 

The  gall  bladder  and  cystic  duct  are  formed  by  a  diverticulum  from  the  common  bile 
duct,  which  appears  in  the  second  month. 

In  the  elasmobranch  fishes,  and  in  amphibia,  there  is  only  a  single  hepatic  diverticulum. 
The  anastomosing  cell-cylinders  which  sprout  from  this  are  not  solid,  but  hollow,  with  a 
narrow  lumen,  and  the  liver  has  from  the  first  and  retains  permanently  the  character  of  a 
compound  gland  formed  of  anastomosing  tubules.  In  reptiles  the  cylinders  also  have  from 
the  first  a  narrow  lumen.     In  birds  and  mammals  the  cylinders  are  solid,  as  in  man. 

As  the  liver  grows,  it  projects  on  either  side  into  the  pleuroperitoneal  cavity.  The 
mesoblast  which  unites  it  to  the  antc-rior  wall  of  the  abdomen,  becomes  thinned  out  to  form 


THE    PANCREAS.  113 

the  suspensory  ligament.  Tliat  wliicli  unites  it  to  the  ventral  wall  of  the  stomach  and  duo- 
denum also  becomes  thinned  out  ;  it  forms  the  small  or  },'astro-hepatic  omentum.  The  liver  is 
at  first  an  exactl}'  symmetrical  or;,'an,  the  rif,'-ht  and  left  loljes  being  etjual  in  size  and 
Bymmetrically  jdaced.  After  tlie  fourth  month  the  ri{,'ht  lobe  bet,'-ins  relatively  to  increase  in 
size,  and  at  birtli  the  proportion  of  this  to  the  left  lobe  is  as  I'G  to  1.  The  liver  also  at  first 
grows  very  rapidly,  so  that  by  the  second  month  it  nearly  fills  the  abdomen,  and  causes  a 
well  marked  prominence  on  the  ventral  aspect  of  the  embryo.  At  this  time  it  is  calculated  to 
constitute  m-arlj'  one  half  the  weight  of  the  body.  Tlic  proportion,  liowever,  gradually 
decreases,  until  at  term  the  relative  weight  of  the  liver  to  the  whole  bodj'  is  as  1  to  18.  The 
further  changes  which  the  blood-vessels  which  pass  to  the  liver  undergo  will  be  considered 
with  the  development  of  the  venous  system. 

The  pancreas  is  developed  as  a  hollow  hypoblastic  diverticulum  from  the  dorsal 
wall  of  the  duodenum  opposite  the  hepatic  diverticula,  and  somewhat  later  than  these 
(fig.  125,  B,  C,  D,  P).  This  hollow  process  grows  into  the  mesogastrium  or  gastro- 
duodonal  mesentery,  which  at  this  time  is  well  developed,  and  ramifies  within  this, 
producing  by  its  oli-shoots  the  ducts  and  alveoli  as  with  other  compound  acinous 
glands.  As  the  duodenal  loop  becomes  formed,  and  this  and  the  pyloric  end  of  the 
stomach  are  turned  over  towards  the  right  side,  the  pancreas  loses  its  median  sym- 
metrical position,  and  with  the  mesentery  which  encloses  it  now  lies  across  the  back 
of  the  abdomen.  This  is  the  condition  in  which  the  gland  is  found  in  most  mammals. 
But  in  man,  with  the  fusion  of  the  mesogastrium  (great  omentum)  to  the  transverse 
mesocolon,  the  posterior  layer  of  the  mesenteric  fold  which  encloses  the  pancreas 
becomes  absorbed  (Toldt),  and  the  gland  becomes  fixed  across  the  back  of  the 
abdomen,  and  is  now  apparently  altogether  behind  the  peritoneum  (see  fig.  129). 


RECENT    LITERATURE, 

Bemmelen,  J.  F.  van,  Entwilckeling  en  metamorphose  der  kiemv  of  viceral-spalten  en  der  aorta- 
bogen  bij  embryonen  van  Tropidonotus  natrix  en  LaceHa  muralis,  Kin.  Akad.  v.  Wet.  Amsterd.  Afd. 
Natuusk.,  1885  ;  Die  Visceraltaschen  u.  Aortenbogen  bei  Reptilien  u.  Vogeln,  Zool.  Anzeiger,  1886  ; 
Die  Halsgegend  der  Reptilien,  Zool.  Anz.,  1887. 

Bonnet,  R.,  Ueber  die  Entivicldung  der  Allantois  und  die  Bildung  des  Afters  bei  den  Wieder- 
Icauern  und  iiber  die  Bedeutung  der  Primitivrinne  und  des  Primitivstreifs  bei  den  Embryonen  der 
Sdugethiere,  Anat.  Anzeiger,  1888. 

Born,  G.,  Ueber  die  Derivate  der  embryonalen  Schlundbogen  und  Schlundspcdten,  Arcbiv  f. 
mikr.  Anat.,  Bd.  xxii.,  1883. 

Cadiat,  Dm  developi2)ement  desfentes  et  arcs  brancliiaux  chez  I'embryon,  Journal  de  I'anat.,  kz., 
1883. 

Chievitz,  J.  C,  Beitrdge  zur  EntwicUungsgeschichte  der  Speicheldriisen,  Arch.  f.  Anat.  u. 
Physiol.,  Anat.  Abtheil.,  1885. 

Demon,  F.,  Developpement  de  laportion  sousdiap>liragmatique  du  tube  digestif.  Lille.  1SS4. 

Dohrn,  A.,  Vie  Thyroidea  bei  Petromyzon,  Amphioxus  u.  Timicaten,  Mitth.  aus  der  zool. 
Station  z.  Neapel,  1886. 

Fischelis,  Ph.,  Beitrdge  zur  Kenntniss  der  EntwicklungsgescJiichte  der  Gl.  thyreoidea  u.  Gl. 
thymus,  Arch.  f.  mikr.  Anat.,  Bd.  xxv.,  1885. 

His,  "W.,  Ueber  den  Sinus  pyrcKcervicalis  und  die  Thymusanlage,  Archiv  f.  Anat.  u.  Physiol., 
Anat.  Abth.,  1886  ;  Zur  Bildungs(jcschichte  der  Lungen  beim  mensehlichen  Embryo,  Archiv  f.  Anat. 
und  Physiol.,  Anat.  Abtheilung,  1887  ;  Schlundspalten  u.  Thymusanlage  {Brief  an  F.  Mall),  Arch, 
f.  Anat.  u.  Physiol.,  Anat.  Abth.,  1889. 

Kastschenko,  N.,  Das  Schicksal  der  embryonalen  Schlundspalten  bei  Sdugethieren,  Archiv  f. 
niikrosk.  Anat.,  Bd.  xxx.,  1887  ;  Das  Sc/dundsjjaltengebiet  des  Hiiknchens,  Arch.  f.  Anat.  u.  Phys. 
AnuL.  Abth.,  1887, 

Liiessner,  E.,  Ein  Beitrag  zur  Kenntniss  der  Kiemenspalten  und  ihrer  Anlagen  bei  amnioten 
Wirbelthieren,  MorphoJog.  Jahrbuch,  Bd.  xiii.,  1888. 

Mall,  F.  P.,  Entwicklung  der  Branchialbogen  und  Spalten  des  Hiihnchens,  Arch.  f.  Anat.  ii. 
Physiol.,  Anat.  Abth.,  1887  ;  The  branchial  clefts  of  the  dog,  with  special  reference  to  the  origin  of 
the  thymus  gland.  Studies  from  the  Biol.  Laboratory  of  John  Hopkins  University,  iv. ,  1888. 

Meuron,  P.  de,  Recherches  sur  le  developpement  du  thymus  et  de  la  glande  thyroide,  Recueil 
zool.  Suisse,  iii.,  1886  ;  Sur  le  developpement  de  l' ossophage,  Compt.  rend.,  1886. 

Minot,  Ch.  S.,  Evolution  of  the  Lungs,  Proceed,  of  the  Zoolog.  Society  of  Loudon,  1886. 

Ostroumoff,  A. ,  Ueber  den  Blastoporus  u.  d.  Schwanzdarm  bei  Eidechsen  u,  Sclachiern,  Zool. 
Anzeiger,  1889. 

Philip,  E.  W.,  Beitrdge  zur  Lehre  ilber  die  Entwicklung  der  Trachea,  Mitth.  aus  d.  embryol. 
lust.  d.  Univers.  Wien,  Bd.  ii.,  1883. 


114  RECENT    LITERATURE. 

Piersol,  Gr.  A. ,  Ueber  die  EntwicMung  der  embryoncUen  ScMundspalten  und  ihre  Dcrivate  bei 
Saugethieren,  Zeitschr.  f.  wiss.  Zool,  Bd.  xlvii.,  1888. 

ilabl,  C,  Zur  Bildungngeschichte  des  Halses,  Prager  medic.  Woclienschr. ,  1886  u.  1887. 
Retterer,  E. ,  Du  developpement  de  la  rdgion  anale  des  mammiferes,  G.  r.  de  la  society  de  biologic, 

1890. 

Bobinson,  A.,  Observations  on  the  earlier  stages  in  the  development  of  the  lum,gs  of  rats  and 
mice,  Journal  of  Anatomy  and  Physiology,  1889. 

Sch-wink,  T.,  Ueber  den  Zwischenkiefer  und  seine  Nachbarorgane  bei  Saugethieren,  1888. 

Stieda,  Untersuchungen  ueber  die  Entwickl.  der  GlanduLa  thymus,  Glandula  thyoidea  und  GlandvZa 
earotica.     Leipzig,  1881. 

Swartz,  D.,  Untersuchungen  des  Schwanzendes  bei  den  Embryonen  der  Wirbelthiere,  Zeitsch.  f. 
wiss.  Zool.  xlviii.,  1889. 

Toldt,  C,  Bail  u.  Wachsthumsverdnderungen  der  Gekrose  des  menschlichen  Darmhanales, 
Wiener  Denkscliriften,  1879  ;  Die  Barmgekrose  u.  Netze  im  gesetzmdssigen  u.  im  gesetzwidrigen 
Zustand.     Ibid.,  1889. 

Usko-w,  N. ,  Bemerkungen  zur  Entwicklunqsgeschichte  der  Leber  und  der  Lungen,  Archiv  f. 
mikrosk.  Anatomie,  Bd,  xxii.,  1883. 

Wolfler,  A.,  Ueber  die  EntwicU.  u.  den  Bau  der  Schildrilse.     Berlin,  1880. 


DEVELOPMENT    OF    'JHE    L'KINAUY    AND    GENERATIVE    ORGANS.       115 


DEVELOPMENT    OF    THE    URINARY    AND    GENERATIVE    ORGANS. 

Tlie  urinary  aiul  u-encratiw  ui'uans  origiiiate  in  cuiiiiccti»in  witli  the  i/ilcnnet/iale 
.ccll-nuis><,  a  jiortion  of  mesublast  wliich  is  secu  in  sections  of  the  early  embryo  lying 


Fig.  136. — Part  of  a  transverse  section  of  a  chick  embryo  of  2  pats,  6  HorRs.  (Kolliker.)  -f* 

itw,  protovertebra  ;  mp,  lateral  mesohlast  ;  dfp,  splancbnopleuric  mesoUast  ;  Iqj.  .somatopleuric 
mesoblast  :  ]),  pleuro-peritoneal  cleft  (ccelom)  ;  u-g,  'Wolffian  duct ;  wk,  part  of  intermediate  cell- mass 
from  which  AVolffian  bodj'  will  become  developed. 


between  the  paraxial  mesoblast  and  the  pleuro-peritoneal  cleft,  and  abuttino-  against 
the  external  epiblast  (fig.  39,  p.  37). 

Fig.   137. — Section   through   an   external   GLOMERrLUs 

OF    THE    pronephros    FROJI  A    CHICK    OF    ABOUT    4    DAYS' 

INCUBATION.     (Balfour. ) 

gl,  glomerulus  ;  ge,  peritoneal  epitl>elium  ;  Wd,  Wolffian 
duct  ;  (10,  aorta  ;  vie,  mesentery. 

Some  of  the  cells  of  this  intermediate  cell-mass 
become  differentiated  into  a  longitudinally  run- 
ning cord,  which  subsequently  acquires  a  lumen, 
and  is  then  known  as  the  Wolffian  dud  (from  its 
discoverer,  Caspar  Friedrich  Wolff)  (fig.  136,  icg). 
Posteriorly  the  duct  opens  into  the  cloaca.  The 
anterior  part  of  the  duct  becomes  connected  with 

inyaginations  of  the  peritoneal  epithelium,  between  which  vascular  glomeruli  project 
freely  into  the  peritoneal  cavity  (fig.  137).  These  glomeruli  constitute  the  head 
kidney,  fore-kuhiey  or  j^i'onepliros}    Along  its  inner  side,  somewhat  fui'ther  back- 

^  Hertwig.  According  to  Balfour  and  Sedgwick,  these  glomeruli  form  the  ant-erior  part  of  the 
Wolffian  body,  and  the  head  kidney  is  represented  by  the  MuHerian  invaginations  referred  to  later  on 
(seep.  122  and  fig.  145). 

I  2 


116      DEVELOPMENT    OF    THE    URINARY    AND    GENERATIVE    ORGANS. 

wards,  a  series  of  transversely  coursing  tubes  becomes  developed  in  the  intermediate- 
cell-mass.  These  tubes  are  connected  for  a  time  with  other  involutions  of  the 
peritoneal  epithehum  (fig.  141),  but  subsequently  lose  their  connection  with  that. 


Fig.  138. — Diagrams  of  the  ARRANGEiiENi  of  the  urinary  and  genital  organs  in  elasmobranchs. 

(Balfour.) 

A. — Diagram  of  the  PRiiiiTivE  condition  of  the  kidney  in  an  elasmobeanch  embryo. 

pd,  segmental  duct  ;  opening  at  o,  into  the  body  cavity  and  at  its  other  extremity  into  the  cloaca  ; 
X,  line  of  separation  between  the  Wolffian  duct  above  and  the  Mldlerian  duct  below  ;  st,  segmental  tubes, 
opening  at  one  end  into  the  body  cavity  and  at  the  other  into  the  segmental  duct. 

B. — Diagram  of  the  arrangement  of  the  trino-genital  organs  in  an  adult  female 

elasmobranch. 

m.d,  Mllllerian  duct  ;  w.d,  Wolffian  duct  :  s.t,  segmental  tubes  ;  five  of  them  are  represented  witli 
ipenings  into  the  body  cavity,  and  five  posteriorly  correspond  to  the  metanephros  ;  ov,  the  ovary  ;  cZ, 
ureter. 

C. — Diagram  of  the  arrangement  of  the  urino-genital  organs  in  an  adult  male  elasmobranch. 

m.d,  rudiment  of  Mldlerian  duct  ;  ic.d.  Wolffian  duct,  serving  at  vd  as  vas  deferens  ;  s.t,  segmental 
tubes,  two  rei>resented  with  openings  into  the  body  cavity  ;  d,  ureter  ;  t,  testis ;  nt,  canal  at  the  base 
of  the  testis  ;  V.E,  vas  efiferentia  ;  Ic,  longitudinal  canal  of  the  Wolffian  body. 


epithelium,  and  acquiring  glomeruli  at  one  part,  at  another  part  open  into  the 
Wolffian  duct.     They  form  the  mid-kidmy,  Wolffian  hocly  or  mesonephros,  which 


THE    WOLFFIAN'    DUCT    AND    BODY. 


117 


presondy  projects  as  a  distinct  vascular  organ  along  the  dorsal  pirt  of  the  peritoneal 
cavity  on  either  side  uf  the  mesentery.  8ubse(iuently  anotlier  duct  becomes  deve- 
loped along  the  outer  side  of  the  Wolttian  body,  along  which  it  runs  backwards  to 
open  also  into  the  cloaca  :  in  fiont  it  communicates  with  the  pleuro-peritoneal  cavity 
by  one  or  more  funnel-shaped  ai)ertures  (tig.  143,  z).  This  is  the  MiiUeiian  dud,  so 
named  after  Jnhannes  Midler  ;  in  some  of  the  lower  vertebrates  it  arises  in  common 
with  the  "WoUHan  duct.  From  the  lower  end  of  each  Wolttian  duct  a  hollow  pro- 
trusion (tig.  12."),  C  and  D,  N)  gi-ows  upwards  into  a  mass  of  mesoblast  continuous 
with  that  of  the  Wolttian  body  ;  with  the  branches  of  this  protrusion  glomeruli  and 
convoluted  tubes  also  become  connected,  and  thus  the  permanent  kidneij  {hind-hUhiei/, 
metitncplirvs)  is  produced.  Lastly,  the  coelomic  epithelium  covering  the  inner  side  of 
the  AVolttiau  body  becomes  thickened  (tig.  14:3,  <'/),  and  within  it  are  found  larger 
cells,  from  which  the  generative  products  in  both  sexes  (ova  and  spermatozoa)  are 
eventually  derived.  This  epithelium  is  accordingly  known  as  the  fjerminalepUheUum. 
The  duct  of  Miiller  becomes  in  the  female  the  oviduct  or  Fallopian  tube  ;  in  the 
male  it  becomes  atrophied.  The  AVolttian  duct  in  the  male  becomes  the  epididymis* 
and  vas  deferens  ;  while  the  vasa  efferentia  and  tubes  of  the  rete  testis  are  formed 
as  outgrowths  fi"om  the  Wolfiian  body ;  in  the  female  these  parts  have  no  ijermanenL 
tiinction. 

The  head  kidney,  although  permanent  and  functional  in  fishes,  is  only  a  rudi- 
jnentary  organ  in  the  embryo  of  higher  vertebrates,  and  soon  disappears.  The 
AVolttiau  body  is  well  developed  in  all  vertebrates  ;  in  fishes  and  amphibia  it  is  an 
important  part  of  the  permanent  urinary  apparatus,  and  also  serves  to  cany  away 
the  male  sexual  products  (tig.  138).  In  higher  vertebrates  (amniota)  it  no  longer 
continues  to  perform  excretory  functions,  but  still  supplies  the  efferent  apparatus  of 
the  testis. 

The  details  of  the  development  of  these  parts  may  next  be  considered. 

The  Wolffian  duct  and  body. — The  commencement  of  the  AVolttian  duct  is 

seen  at  a  very  early  period  of  development  (second  day  in  the  chick,  eighth  day  in  the 

rabbit)  as  a  thickening  of  the  intermediate  cell-mass  in  the  anterior  region  of  the 

trunk  (fifth  somite)  {?ig.  139,  ^Yd).    The  outgrowth  projects  towards  the  epiblast,  and 


yric^^ 


±^£^/T2/> 


Tig.  139. — Trassvekse  section  of  ax  embryo  chick  of  thikty-six  hours.     ^\^    (E.  A.  S.) 

n.c,  medullary  tube  ;  p,  protovertebra  ;  ep,  epiblast;  vie,  lateral  mesoblast  split  into  splanchnopleure 
and  somatopleure  ;  ae,  pleuro-peiitoneal  cavity  between  them  :  ca',  cavity  of  protovertebra.  continuous 
■on  the  right  side  with  the  lateral  mesoblastic  cleavage;  W.d.,  Wolffian  duct;  W.h.,  mesoblast  ot 
Wolffian  body  ;  ch,  notochord. 


developes  from  before  backwards ;  a  solid  cord  of  mesoblast  thus  becomes  formed,  which 
.gradually  becomes  detached  from  the  remainder  of  the  intermediate  cell-mass,  lying 
close  to  the  epiblast  (fig.  142,  ic.d.).  Soon  after  it  is  thus  formed,  a  lumen  appears  in 
it  and  extends  both  forwards  and  backwards.  The  posterior  end,  which  is  still  solid, 
is  presently  found  to  be  attached  to  the  epiblast,  and  apparently  continues  to  grow 
backwards  along  and  at  the  expense  of  the  epiblast  until  it  reaches  the  posterior  end 


118 


THE   WOLFFIAN    DUCT   AND    BODY. 


of  the  body,  where  it  becomes  detached  from  the  epiblast,  and  is  connected  with  and' 
opens  into  the  hind- gut  (cloaca). 

I  have  here  followed  what  has  appeared  to  me  the  most  probable  account  of  the  orio-in  of 
the  duct  (Martin,  Strahl),  but  it  is  right  to  state  that  in  the  opinion  of  some  observers 
(Hensen,  Spee,  Flemming)  the  formation  and  growth  of  the  duct  in  connection  with  the 
epiblast  is  primary,  especially  in  mammals,  and  the  duct  is  originally  formed  by  a  longitudinal 
thickening  and  involution  of  the  epiblast,  which  only  secondarily  becomes  connected  with  the 
intermediate  cell-mass.  Compare  also  Haddon,  Origin  of  segmental  duct,  Proc.  Roy.  Dublin 
Society,  Vol.  V. 

In  teleosteans  (Rosenburg)  and  amphibia  (Gotte)  the  Wolffian  duct  has  been  described  as 
developing  in  the  form  of  a  longitudinal  groove-like  invagination  of  the  somatopleural  meso- 
blast  (Balfour,  Comp.  Emb.,  vol.  ii.,  pp.  .580,  .582),  but  more  recent  researches  appear  to  indi- 
cate that  in  these  animals  also  the  epiblast  may  be  concerned  in  its  formation. 

The  Wolffian  body  developes  in  the  intermediate  cell-mass  between  the  Wolffian 
duct  and  the  body-cavity  as  a  series  of  transverse  tubes  which  lie  at  right  angles  to 
the  course  of  the  Wolffian  duct,  and  open  into  it  at  regular  intervals.     The  usual 


Fig.  140.— Transverse  sec- 
tion OF  THE  TRUNK  OF  A 
cat  embryo,  showing  the. 
vesicular  stage  of  the, 
avolffian  tubules. 

(E.   A.   S.). 

m.p.,  muscle  plate  ;  ao, 
aorta ;  m.ff.,  mid-gut  ;  am,. 
amnion  ;  w,  vesicle  of  Wolffian 
body ;  w.cl.,  Wolffian  duct ;. 
cos,  ccelom. 


mode  of  formation  of 
these  tubes — which  are 
sometimes  termed  seg- 
mental  tubes  —  appears 
to  consist  in  the  accu- 
mulation at  regular  intervals,  corresponding  with  the  somites,  of  rounded  masses 
of  mesoblast  on  the  mesial  or  ventral  side  of  the  Wolffian  duct  (fig.  142,  iv.h.), 
which  masses  become  afterwards  hollowed  out  so  as  to  form  small  vesicles,  at 
first  isolated,  but  afterwards  growing  towards  and  opening  into  the  Wolffian 
duct  (fig.  140).i  Corresponding  with  these  vesicles  there  become  formed  invagi- 
nations of  the  epithelium  of  the  body-cavity  (fig.  141,  st),  which  is  thickened 
along  the  inner  side  of  the  Wolffian  projection,  and  grows  at  regular  intervals 
towards  the  vesicles.  These  ingrowths  may  at  first  communicate  by  funnel-shaped 
openings,  which  in  some  lower  vertebrates  are  lined  by  ciliated  epithelium,  with 
the  body-cavity,  but  the  openings  in  higher  vertebrates  become  dosed  again  before 
communication  with  the  Wolffian  duct  is  established.  Finally,  the  connection 
between  the  Wolffian  tubes  and  the  peritoneal  epithelium  is  completely  severed,  and 
the  condition  of  simple  or  curved  transverse  tubes,  blind  at  their  inner  ends  and 
opening  at  their  outer  ends  into  the  Wolffian  duct,  is  produced  (fig.  142,  B).  After 
a  time  the  blind  extremities  are  seen  to  be  enlarged  and  spoon-shaped,  and  glomeruli 

1  According  to  v.  Wijhe  the  hollow  condition  is  the  primary  one  in  elasmobrancbs,  and  the  cavity  of 
each  vesicle  represents  an  intermediate  part  of  the  ccelom  of  the  segment  (meso-coelom),  the  dorsal 
coelom  being  represented  by  the  cavity  of  the  proto-vertebra  and  the  ventral  coelom  by  the  pleuroperi- 
toneal  space.  I  have  myself  observed  this  condition  of  a  hollow  intermediate  cell-mass  communicating: 
on  the  one  hand  with  the  cavity  of  the  protovertebra  and  on  the  other  with  the  cleft  of  the  lateral 
mesoblast,  in  a  chick  of  36  hours  (see  fig.  139). 

In  mammals  the  Wolffian  vesicles  are  more  numerous  than  the  segments. 


THE    WOLFFIAN    DUOT    AND    iiuDY. 


119 


are  observed  developiuLj:  in  the  bowl  of  the  spoon   from  mesoblast   cells,  which 
presently  become  entirely  enclosed  by  the  end  of  the  tube.     Subsequently  a  secund 


Fig.  141. — Traxsverse  section  through  the  trunk  of  a  duck  embryo  vhte.  about  twenty-focb 

MESOBLASTIC    SOMITES.       (BalfoUF.) 

am,  amnion ;  so,  somatopleure  ;  sp,  splanclinopleure  ;  wd,  "Wolfl&an  duct ;  st,  segmental  tube  with 
peritoneal  involution  ;  ca.v,  cardinal  vein  ;  ni.s,  muscle-plate  ;  s^p.ff,  spinal  ganglion  ;  sj^.c,  spinal  cord  ; 
ch,  notochord  ;  ao,  aorta  ;  hi/,  hypoblast. 

and  a  third  set  of  tubes  become  developed  in  a  similar  manner,  but  without  perito- 
neal invaginations,  and  also  open  directly  into  the  Wolffian  duct.     Lastly,  other  tubes 


cofn^t^^- 


Fig.  142. — Transverse  sections  of  sheep  embryos,   showing   two   stages  in  the  development  of 

THE    WOLFFIAN    BODY.        (BonUCt. ) 

w.d,  Wolffian   duct;    v:.h,  Wolffian  body;   -p.v,   protovertebra  ;    cl>,  notochord;    a.c,  neural  canal; 
am,,  amnion  ;  ao,  aorta  ;  i,  intestine;  y.s,  yolk-sac. 

with  glomeruli  become  formed  between,  and  open  into  those  which  are  already 
connected  with  that  duct.  All  these  tubes  are  short  and  straight  when  first  deve- 
loped, but  afterwards  lengthen  and  become  converted  into  convoluted  uriniferous 
tubes,  which,  like  those  of  the  permanent  kidneys,  begin  in  a  dilated  extremity 
enclosing  a  tuft  of  capillary  blood-vessels  (glomerulus),  which  are  supplied  by 
branches  of  the  primitive  aortre. 


120 


THE    WOLFFIAN   DUCT    AND    BODY. 


When  completely  formed,  the  Wolffian  bodies  are  seen  on  opening  the  abdomen 
of  the  embryo  as  long  prominent  vascular  organs  projecting  into  the  peritoneal 
cavity  on  either  side  of  the  intestine,  and  showing  in  section  numerous  Malpighian 
corpuscles  and  uriniferous  tubules  variously  cut  (fig.  143). 

Soon  after  having  attained  its  complete  condition  of  development,  the  Wolffian 
body  begins  to  undergo  atrophic  changes.     These  proceed  much  further  in  the 


-c' 


Fig.   143.  —  Transverse    section    of    the 

WOLFFIAN    BODY    OF    THE    CHICK    ON    THE 

FOURTH  DAY.     (Waldejer. ) 

VI,  mesentery  ;  Z,  hody  wall ;  a',  thickeLed 
epithelium  from  whicii  the  involution  of  the 
anterior  part  of  the  illillerian  duct  2,  is 
taking  place  ;  a,  thickened  germinal  epithe- 
lium in  which  are  seen  primitive  ova,  0  ; 
E,  modified  raesoblast  which  will  form  the 
stroma  of  the  ovary  ;  WK,  tubules  of  Wolffian 
body  variously  cut  ;  y,  "Wolffian  duct.  Two 
glomeruli  are  shown  in  the  Wolffian  body. 


female  sex  than  in  the  male,  but  the 
tubules  of  the  organ  do  not  entirely 
disappear  in  either  sex.  In  the 
female  they  form  the  rudimentary 
organ  which  is  known  as  the  par- 
ovariicm  {ejioophoron  of  Waldeyer), 
while  the  main  tube  of  that  struc- 
ture represents  a  remnant  of  the 
Wolffian  duct.  But  in  many  animals, 
e.g.,  the  sow,  the  Wolffian  duct  re- 
mains as  the  duct  of  Gartner,  a 
strong,  slightly  undulated  tube,  which  is  traceable,  at  first  free  in  the  broad  ligament 
of  the  uterus,  and  lower  down  becoming  incorporated  with  the  wall  of  the  uterus 
and  vagina,  upon  which  last  it  becomes  lost.  Traces  of  this  tube  can  sometimes  be 
seen  in  sections  across  the  body  or  cervix  of  the  adult  human  uterus,  and  even  lying 
in  the  Avail  of  the  vagina. 

In  the  male  the  Wolffian  duct  forms  the  tube  of  the  epididymis,  the  vas  deferens, 
and  the  ejaculatory  duct ;  the  seminal  vesicle  being  formed  as  a  diverticulum  from 
its  lower  part.  The  coni  vasculosi  and  tubuli  eflFerentes  are  in  all  probabihty  formed 
by  the  persistence  of  some  of  the  tubules  of  the  Wolffian  body.  The  Malpighian 
corpuscles  of  these  tubules  have  long  disappeared,  but  j)revious  to  their  disappearance 
solid  columns  of  epithelial  cells,  afterwards  becoming  tubules,  grow  from  the  walls  of 
those  corpascles  towards  the  germinal  epithelium  (fig.  153),  where,  in  the  male,  they 
become  continuous  with  and  enclose  cells  derived  from  that  epithelium  (which 
subsequently  form  the  epithelium  of  the  seminiferous  tubes),  and  thus  produce  the 
walls  of  the  seminiferous  tubules  and  the  rete  testis.  In  the  female  sex  there  is  also 
a  growth  of  solid  cellular  columns  towards  the  germinal  epithelium,  but  no  connec- 
tion becomes  established  between  them,  and  the  columns  do  not  become  tubular. 
The  organ  of  Giraldes  and  the  vasa  aberrantia  of  Haller  are  probably  the  remains 
of  one  or  more  Wolffian  tubules. 

Suprarenal  capsules. — These  org-ans  are  intimately  connected  in  their  development  with 
the  Wolffian  bodies.  According-  to  the  observations  of  Welclon  some  of  the  cellular  columns 
which  grow  from  the  Malpighian  corpuscles  of  the  upper  part  of  the  Wolffian  body  towards 
the  germinal  epithelium  give  offsets  which  pass  njDwards  towards  the  inferior  vena  cava,  and 
there  become  developed  into  the  cortical  substance  of  the  suprarenal  capsules.  (Mihalkovics, 
on  the  other  hand,  states  that  the  strands  of  cells  which  grow  from  the  upper  part  of  the 


SUPRARENAL    CAPSULES. 


121 


"Wolffian  lK)»ly  to  take  part  in  the  formation  of  the  suprarenal  capsules  have  been  derived  by 
proliferation  from  the  -rerminal  tpithelium.j  It  had  lon;r  been  believed  that  the  two  parts  of 
these  or-jans.  cortical  and  medullary,  are  sejiarate  in  ori;rin  :  the  former  being  derived,  as  was 
thou^rht,  fronx  cells  which  are  of  mesoblastic  oritrin.  the  latter  ^>eing•  developetl  in  connection 
with   the  sympathetic  j^anglia.      In  elasmobrauchd  and  Kome  other   lower  vertebraiei*,  they 


Pig.    144.— Internal  organs  of  a  fkmale  hcmax  foetis  of  3^  inches  long,  or  about  14  weeks. 

Macnified  (from  Waldeyer). 

o,  the  ovary  full  of  primordial  ova  ;  c,  tubes  of  the  upper  part  of  the  Wolffian  body  forming  the 
epoophoron  (parovarium  of  Kobelt)  ;  W,  the  lower  part  of  the  Wolffian  body  forming  the  paroophoron 
of  His  and  "\Valdeyer ;  w,  the  "Wolffian  duct  ;  JI,  the  Miillerian  duct,  with  its  upper  opening  already 
timbriated. 

Fig.  145. — Internal  genital  organs  of  a  male  HtJSiAN  embrto  of  3^  isches  losg  (from  Waldeyer). 

t,  body  of  the  testicle  with  seminal  canals  formed ;  e,  epididymis,  or  upper  part  of  Wolffian  body ; 
IT,  Wolffian  body,  lower  part,  becoming  paradidymis  or  organ  of  Giraldes  ;  w',  Wolffian  duct,  becoming 
vas  deferens  ;  ff,  gubernaculum. 


B 


Fig.  146.— Two   figures  exhibiting  a  comparison  between  parts  of  the  generative  organs  15 
THE  TWO  sexes  (fiom  FarTc,  after  Kobelt). 

A. AdCLT    OVARY,    PAROVARIUM    AND    FALLOPIAN    TUBE. 

rt,  «,  Epoophoron  (parovarium)  formed  from  the  upper  part  of  the  Wolffian  body  ;  b,  remains  of  the 
uppermost  tubes,  sometimes  forming  hydatids  ;  c,  middle  set  of  tubes  :  d.  some  lower  atrophied  tubes  ; 
c,  atrophied  remains  of  the  Wolffian  duct  :  /.  the  terminal  bulb  or  hydatid  ;  h,  the  Fallopian  tube, 
originally  the  duct  of  iMidler  ;  (,  hydatid  attached  to  the  extremity  ;  I,  the  ovary. 

B. — The  adult  testis  and  epididymis. 

rt,  a,  convoluted  tubes  in  the  head  of  the  epididymis  developed  from  the  upper  part  of  the  Wolffian 
body  ;  b  and  /.  hydatids  in  the  head  of  the  epididymis  ;  c,  coni  vasculosi  i  d,  vasa  aberrantia  :  h, 
remains  of  the  duct  of  iliiller  with  i,  the  hydatid  of  Slorgagni,  at  its  upper  end  ;  I,  body  of  the  testis. 


132  THE    Mt)LLERIAN   DUCT. 

consist  throughout  life  of  two  separate  portions,  one  median  and  single,  the  other,  dei-iTed' 
from  the  sympathetic  ganglia,  paired  ;  in  birds,  reptiles,  and  mammals  these  distinct  'portion^ 
are  combined  into  the  two  paii-ed  organs  (Balfour).  But  in  these  also,  as  has  been  shown  by 
Mitsukuri  for  mammals,  the  medullary  or  nervous  part  is  at  first  distinct  and  outside  the- 
cortical,  into  which  it  gradually  insinuates  itself,  retaining,  however,  its  connection  with  the- 
neighbouring  sympathetic  ganglia. 

The  permanent  kidneys  arise  (1)  as  protrusions  from  the  posterior  end  of  the 
Wolffian  ducts  (see  fig.  125,  C  and  D,  i\^),  which  grow  forwards  towards  the  lower  part 
of  the  Wolffian  bodies,  and  form  the  ureters  and  the  collecting  tubules  of  the  kidney  ; 
(2)  from  a  portion  of  the  intermediate  cell-mass  situated  posterior  to  the  Wolffian 
body,  and  within  which  convoluted  tubes  and  Malpighian  corpuscles,  and  eventually 
the  remaining  parts  of  the  uriniferous  tubules  become  developed.  But  before  these 
changes  occur  in  this  intermediate  cell-mass,  it  shifts  its  position  relatively  to  the 
Wolffian  body,  eventually  coming  to  lie  above  and  behind  that  organ.  The  con- 
voluted tubes,  with  their  Malpighian  corpuscles,  appear  to  be  developed  independently 
of  the  ureter  and  collecting  tubes,  as  in  the  case  of  the  Woh!ian  tubules  and  the 
Wolffian  duct,  a  communication  between  them  being  only  subsequently  established. 

The  glomeruli  are  apparent  in  the  eighth  week  in  the  human  foetus.  In  the 
third  month  the  papillas  are  formed,  and  in  the  fourth  month  the  loops  of  Henle  are 
seen.  The  tubes  are  wider  in  the  foetus  than  in  the  adult  ;  the  expansion  of  the 
kidney  as  growth  advances  must  therefore  be  due  mainly  to  an  increase  in  length  of 
the  tubules,  since  new  tubules  and  glomeruli  do  not  appear  to  be  formed.  The 
human  kidney  is  at  first  lobulated,  the  lobules  corresponding  in  number  to  the 
Malpighian  pyramids,  but  by  the  end  of  the  first  year  after  birth,  the  kidneys  have 
usually  nearly  lost  their  lobulated  appearance. 

The  nrinary  bladder  is  formed  by  a  spindle-shaped  dilatation  of  the  stalk  of 
the  allantois  (second  month).  The  upper  pole  of  the  spindle  extends  as  the  urachiis- 
into  the  umbilical  cord  ;  it  not  unfreqnently  remains  hollow  for  some  length  within 
the  cord  (Luschka).  The  lower  pole  of  the  spindle  which  passes  towards  the  cloaca 
becomes  the  first  part  of  the  urethra  of  the  male,  and  the  whole  of  the  urethra 
of  the  female.  The  rest  of  the  male  urethra  is  formed  and  enclosed  by  the  folds  of 
integument  which  produce  the  penis  (see  p.  128).  The  ureters,  which  are  originally 
prolonged  from  and  open  into  the  Wolffian  ducts,  subsequently  become  shifted  in 
position,  so  as  eventually  to  open  into  the  enlargement  of  the  allantoic  stalk,  from 
which  the  bladder  is  formed. 

The  Mnllerian  duct. — In  lower  vertebrates,  as  was  shown  by  Balfour  for 
elasmobranchs,  this  duct  takes  origin  by  the  splitting  off  of  the  ventral  part  of  a. 
longitudinal  segmental  or  Wolffian  duct,  the  dorsal  part  remaining  as  the  Wolffian 
duct  proper,  and  receiving  the  segmental  and  uriniferous  tubes,  while  the  ventral  part 
retains  the  funnel-shaped  orifice,  by  which  the  segmental  duct  communicated  ante- 
riorly with  the  body  cavity,  and  comes  to  open  posteriorly  into  the  cloaca  by  an 
orifice  distinct  from  that  of  the  Wolffian  duct  (fig.  138  and  fig.  147).  In  amniotic 
vertebrates,  the  process  of  formation  of  a  Miillerian  duct  is  somewhat  different. 
It  arises  on  the  outer  side  of  the  already  fairly  well  developed  Wolffian  body,  and 
some  little  distance  from  the  anterior  end  of  that  organ,  as  a  thickening  of  the 
peritoneal  epithelium  (fig.  143,  a'),  which  thickening  becomes  invaginated  towards 
the  adjacent  Wolffian  duct,  in  the  form  of  three  successive  funnel-shaped  depressions 
(fig.  148),  somewhat  similar  to  those  which  are  connected  with  the  previously  formed 
Wolffian  segmental  tubes.  The  invaginations  are  connected  together  by  a  con- 
tinuous epithelial  ridge,  forming  a  cord  which  becomes  disconnected  from  the 
peritoneal  cavity  except  at  the  anterior  invagination,  and  subsequently  acquires 
ft  lumen.  The  short  tube  which  is  thus  formed,  soon  begins  to  grow  backward 
as  a  solid  rod  of  cells,  which  comes  in  close  contact  as  it  proceeds  with  the  Wolffian' 


THE    MCLLERIAN    DUCT.  125 

duct  (fiff.  141)).     To  this  duct  it  presently  adheres  intimately,  and  thf-n  continues 
Fig.  147.— Four  sections  thkuich  the  antkkkiu  i-akt  ok  tiik 

SEGMENTAL    IiUCT    OP    A    SCYLLIUM    EMBUYo.       (littlfour.  ) 

The  tij:i:re  shows  how  the  sppiicntiil  duct  hecomes  siilit  into  tlic 
Wolrtian  iluct  (l(irs;illy  and  the  Mullorian  duct  or  oviduct  ventrally  ; 
Wil,  Wolrtian  duct;  od,  Muilciian  duct  or  oviduct;  »(/  (iu  D), 
segiuental  duct. 

to  grow  backwarcis  for  a  certain  distance  as  a  thicken- 
ing of  the  epithelium  of  that  tube,  the  thickening 
becoming  gradually  separated  oft'  from  before  back- 
wards, and  the  lumen  passing  along  it.  Further  back 
it  ceases  to  grow  thus  in  connection  with  the  AVoltiiau 
duct,  but  is  prolonged  as  an  independent  cellular  cord, 
which  lies  in  a  groove  along  the  side  of  the  Wolffian 
duct  (Balfour  and  Sedgwick), 

Entering  the  f/c/ii/al  cord}  the  two  Miillerian  ducts 
lie  at  first  on  the  mesial  side  of  the  corresponding 
"Wtilffian  ducts,  but  lower  down  pass  behind  them  ; 
they  finally  come  again  between  these  ducts,  lying 
close  together,  and,  according  to  Mihalkovics,  approach 
close  to  the  sinus  urogenitalis,  which  by  this  time  is 
formed  out  of  the  ventral  part  of  the  cloaca  (see  p.  128) 

without  actually  opening  into  it  for  some  time.     The  Miillerian  ducts  fuse  together 
below  into  a  single  tube  (second  month);  the  fusion  begins  not  at  the  lower  end,. 


Fig.  148. — Sections  from  the  chick  showing  two  of  the  peritoneal  invaginations  which  give 
RISE  to  the  anterior  PART  OF  THE  MCLLERIAN  DUCT.     (Balfour  and  Sedgwick.) 

gr",    sccoud  invagination  ;    gi-^,    third   invagination  ;    r-,    epithelial    ridge   lietween   them ;     Wd, 
"Wolffian  duct.     These  structures  form  the  pronephros  of  Balfour  and  Sedgwick  (see  note,  p.  115). 

Fig.  149. — Two  sections  from  the  chick 

SHOWING  THE  JUNCTION  OF  THE  TER- 
MINAL SOLID  PORTION  OF  THE  ^McL- 
LERIAN    DUCT    WITH    THE    ^YoLFFIAN 

DUCT.     (Balfour  and  Sedgwick.) 

In  A,  the  terminal  portion  of  the  duct 
is  quite  distinct  ;  in  B  it  has  united 
■with  the  wall  of  the  Wolffian  duct,  md, 
Miillerian  duct ;   Wd,  Wolffian  duct. 

but  a  short  distance  away  from 
this  (fig.  150,  3),  and  proceeds 
both  downwards  towards  the 
future  orifice  and  upwards  for  a 

^  A  name  given  to  the  thickened  mass  of  tissue  which  surrounds  the  Wolffian  ducts  as  they  course 
together  to  the  cloaca  behind  the  stalk  of  the  allantois  (afterwards  the  base  of  the  bladder). 


124 


DEVELOPMENT    OF    THE    OVARY. 


certain  leugtli,  the  amount  of  this  upward  extension  of  the  fused  ducts  varying  in 
different  animals. 

The  united  part  of  the  Miillerian  ducts  afterwards  forms  the  foundation  of  the 
vagina  and  uterus  in  the  female,  and  the  prostatic  vesicle,  or  uterus  masculinus  in 


Fig.    150. — Transverse    sections    of    the 

GENITAL  CORD  IN  A  FEMALE  CALF  EM- 
BRYO. Magnified  fourteen  diameters. 
(KoUiker. ) 

1,  near  tlie  upper  end  ;  2  and  3,  near  the 
middle  ;  4,  at  the  lower  end  ;  a,  anterior, 
p,  posterior  asjject  ;  m,  Miillerian  ducts 
united  or  separate  ;  iv.  Wolffian  ducts. 

the  male  ;  the  upper  or  fore  part  of 
the  Miillerian  duct  disappears  in 
the  male,  in  the  female  it  forms  the 
oviduct  (Fallopian  tube). 

The  hydatids  of  Morgagni  are 
remnant    of    part    of    the  Miillerian 


believed 
dnct. 


to    represent    in    the    ma^e    the 


In  the  human  embiyo  of  the  thii-d  month  the  uterus  is  bifid,  and  it  is  by  the  upward  ex- 
tension of  the  median  fusion  that  the  triangular  body  of  the  uterus  is  produced.  The  bifid 
condition  corresponds  with  the  bicomed  uterus  of  many  animals,  and  the  process  of  fusion 
above  described  explains  the  occasional  malformation  of  a  partial  or  complete  division  of  the 
uterus  and  vagina  into  two  passages.  Up  to  the  fifth  month  there  is  no  distinction  between 
vagina  and  uterus.  Then  the  os  uteri  begins  to  be  seen,  and  the  cervix  uteri  subsequently 
becomes  manifest  as  a  part,  which  is  at  fii-st  thicker  and  larger  than  the  rest  of  the  organ. 

In  some  animals  the  prostatic  vesicle  of  the  male  is  prolonged  into  cornua  and  tubes  like 
the  uterus  of  the  female. 

The  germinal  epithelium. — This  name  was  given  by  "Waldeyer  to  the  thick- 
-eued  epithelium  lying  along  the  inner  side  of  the  Wolffian  projection  (fig.  143,  a). 
The  cells  become  at  first  columnar,  and  then  two,  three,  or  even  several  layers  thick, 
while  at  the  same  time  the  mesoblast  below  them  becomes  increased  in  amount,  and 
thus  a  marked  projection  is  produced,  which  in  some  vertebrates  forms  a  distinct 
ridge — the  genital  ridge.  Amongst  the  (jells  of  the  germinal  epithelium,  some  are 
seen  which  are  larger  and  more  spherical  than  the  others,  these  are  the  primordial 
ova  (fig.  143,  o),  and  occur  in  both  sexes  ;  in  fact,  up  to  a  certain  point,  the  differ- 
ence of  sex  of  the  embryo  is  not  apparent. 

Development  of  the  ovary. — In  the  female  sex  the  germinal  epithelium  soon 
becomes  much  thickened,  and  begins  to  grow  down  into  the  mesoblastic  stroma  in  the 
form  of  columns  of  epithelium  cells,  which  enclose  amongst  them  some  of  the  prim- 
ordial ova.^  These  columns  constitute  the  egg-tahes  of  Pfliiger  (fig.  152).  They 
are  separated  from  one  another  by  mesoblast,  which  grows  towards  and  into  the 
germinal  epithelium  simultaneously  with  the  down-growth  of  the  egg-tubes,  and 
there  is  thus  produced  a  complete  interlocking  of  strands  of  connective  and  epithelial 
tissue,  which  together  constitute  the  ovary.  The  egg-tubes  next  become  broken  up 
into  rounded  groups  or  "  nests "  of  germinal  epithelial  cells,  each  of  which  may 
enclose  one  or  more  primordial  ova.  The  primordial  ova  eventually  develope  into 
ordinary  ova,  two  or  more  frequently  fusing  together  to  form  a  single  ovum  (Balfour), 
while  from  the  remaining  cells  in  the  "nest  "  the  epithelium  of  the  Graafian  follicle 
is  eventually  produced.     In  many  of  the  cell  nests,  primordial  ova  cannot  at  first  be 


1  :Mihalkovics  states  that  the  cells  which  are  to  form  the  follicular  epithelium  first  sink  into  the 
stroma,  and  that  afterwards  the  primordial  ova  follow  them,  and  become  enclosed  by  them. 


DEVELOPMENT    OF    THE    TESTICLE. 


J25 


distinguished,  but  become  formed  subsequently  by  an  increase  in  size  of  one  or  more 
of  tlie  cells.  Tiie  further  changes  which  take  place  in  the  Graafian  follicle  are 
described  with  the  structure  of  the  ovary  (r.  Splanchnology).     The  remainder  of 


Fig.  151. — Traxsvekse  section  throcgh  the  ovart  of  ax  embryo  shark  (scvllicm;,  showing  thc 

GERJI-EPITHELIUM  F0R3IIXG    PRIMITIVE    OVA.      iBtllfour.) 

At  po,   the   germ-epithelium  and  primitive  ova  ;  the  lightly-shaded  part  is  the  ovarian  stroma, 
covered  elsewhere  by  flattened  epitheJium. 


the  germinal  epithelium  which  is  left  covering  the  surface  after  the  formation  of 
the  egg-tubes,  constitutes  the  permanent  epithelium  of  the  ovary. 

Most,  if  not  all.  of  the  permanent  ova  are  produced,  at  least  in  the  human  .subject,  long 
before  birth..     In  the  human  ovary  the  nests  of  cells  which  are  to  form  the  Gmafian  follicle's- 

Fig.  152. — Section  of  the  ovary  of  a 

NEWLY-BORN    CHILD.        HiGHLY   ilAG- 

kified.     (Waldeyer.) 

a,  Germinal  epithelium  dipping  in  at 
b,  to  form  an  ovarian  tube  ;  c,  c,  prim- 
ordial ova  lying  in  the  germ-epithelium  : 
d,  d,  longer  tube  becoming  constricted  so 
as  to  form  nests  of  cells  :  e,  c,  larger  ne.=!ts  ; 
/,  distinctly  formed  follicle  with  ovum 
;.nd  ei^ithelium  ;  g,  (f,  blood-vessels. 

are  more  eciually  diffused  through  the 
substance  of  the  ovary  than  in  most 
animals,  in  many  of  which  the  young- 
follicles  remain  fomiing  a  stratum 
near  the  surface.  In  the  human 
embryo  of  from  four  months  up  to 
the  ijeriod  of  birth,  the  ovary  seems 
to  be  foiTQed  of  little  else  than  a  mass 
of  young'  ova.  closely  surrounded  by 
flattened  cells  of  the  germinal  epithe- 
lium and  constituting  thus  minute 
Graafian    follicles  :    the    amount    of 

stroma  being  at  this  time  relatively  small.     It  has  been  calcixlat«d  that  the  ovaries  mav 
at  this  stage  contain  as  many  as  70,000  primordial  ova. 

Development  of  the  Testicle. — The  germinal  epithelium  does  not  undergo  so 
marked  an  hypertrophy  in  tlie  male  as  in  the  female.  But  it  becomes  thickened, 
and  enlarged  ceils,  corresponding  to  the  primordial  ova  in  the  female,  are  found  m 
it.     Further,  small  strands  of  the  epithelium  dip  down  into  the  subjacent  mesoblast,. 


;,  '-^^^■'p 


126 


DESCENT    OF    THE    TESTICLES. 


which  grows  simultaneously  into  the  epithelium,  and  eventually  cell-nests  are  sepa- 
rated and  included  in  the  mesoblastic  tissue.  Whether  these  nests  are  derived  from 
the  division  of  the  primordial  ova  only,  or  whether  they  also  include  other  cells  of  the 
germinal  epithelium  is  not  clear.  It  would  appear  that  from  these  cell-nests  the 
epithelium  of  the  seminiferous  tubules  is  developed,  although  all  stages  of  the 
process  have  not  been  observed.  The  cell-nests  eventually  become  connected  with 
the  outgrowths  from  the  Wolffian  bodies  (fig.  153,  st),  which  as  already  mentioned, 


Tig.  153.  —Section  of  the  germInal  epithelium  and  adjacent  strojia  in  a  male  chick  embryo. 

(Semon. ) 

g.(p,  germinal  epithelium  forming  a  tliickenecl  ridge-like  projection  ;  ifyr.ov,  primitive  ova  of  various 
sizes,  some  in  the  germinal  epithelium  and  others  somewhat  beyond  the  limit  of  this  epithelium  ;  st, 
strands  of  cells  which  have  grown  from  the  Wolffian  body  towards  the  germinal  epithelium,  and  one  of 
■which  appears  connected  -with  an  enlarged  primitive  ovum. 

form  the  rete  testis  and  the  efferent  tubes  of  the  testicle.  The  reproductive  gland  is 
in  both  sexes  at  first  attached  directly  to  the  Wolffian  body  (fig.  156,  A,  oi),  which 
itself  is  attached  by  a  fold  of  peritoneum  to  the  back  of  the  abdominal  cavity. 
This  fold  becomes  the  mesovarium  or  mesorchium  as  the  case  may  be.  K  band  also 
passes  from  the  Wolffian  body  upwards  to  the  diaphragm,  and  another  fold  contain- 
ing involuntary  muscular  fibres — the  plka  guhernatrix — runs  down  towards  the 
groin  from  the  lower  part  of  the  AVolffian  body  and  the  duct.  This  band,  as  the 
Wolffian  body  becomes  atrophied,  is  found  to  be  attached  to  the  reproductive  organ, 
constituting  the  guhernaculum  testis  in  the  male,  and  the  round  ligament  of  the  ovarij 
ia  the  female  (fig.  15C,  g). 

Descent  of  the  Testicles. — The  testicles  originally  lie  in  the  lumbar  region  of 
the  abdomen.  From  this  j)art  they  become  shifted,  at  first  to  the  internal 
abdominal  ring,  opposite  w^hich  they  are  found  in  the  sixth  month,  and  which  thev 
•enter  in  the  seventh  month,  then  down  the  inguinal  canal  into  the  scrotum,  which 


THE  EXTERNAL  ORGANS. 


127 


they  usually  enter  by  the  eud  of  the  eighth  month.  But  previously  to  this,  a  pouch 
of  peritoneum — the  processus  vayinalis — has  descended  into  the  scrotum  along  the 
abdominal  ring,  pushing  before  it  part  of  the  internal  olilique  muscle  and  the 
aponeurosis  of  the  external  oblique,  which  form  respectively  the  cremasteric  muscle 


Fig.  154. — Diagrams  to  illustrate   the   descent  of  the   testicle  axd  the   f^rmatiux    op   its 

covERiXGS.     (0.  Hertwig.) 

In  A  the  testicle  is  lying  close  to  the  internal  abdominal  ring.  In  B  it  has  passed  into  the  sic  of 
the  tunica  v;iginalis.  1,  skin  of  abdomen  ;  1',  skin  of  scrotum  ;  2,  superficial  abdominal  fascia  ;  2', 
Cooper's  fascia  :  3,  muscular  and  aponeurotic  layer  of  abdominal  wall  ;  3',  cremaster  muscle  and  sper- 
matic fascia  ;  4,  peritoneum  ;  4',  processus  vaginalis  ;  4"  visceral  layer  of  processus  vaginalis  covering 
testicle  ;  t,  testicle  ;  v.d.,  vas  deferens  ;  r,  internal  abdominal  ring. 


and  spermatic  fascia  (tig.  154).  This  pouch,  after  the  descent  of  the  testicle  into 
it,  becomes  shut  off  from  the  abdominal  cavity,  and  forms  the  cavity  of  the  tunica 
vaginahs.  The  descent  of  the  testicle  into  the  scrotum  is  intimately  connected  with 
changes  in  the  gubernaculum.  The  gul^ernaculum  extends,  as  before  mentioned, 
from  the  integument  of  the  groin,  which  aftei-^'ards  forms  the  scrotum,  upwards 
through  the  abdominal  ring  to  the  lower  part  of  the  epididymis.  When  the  pr?)- 
cessus  vaginalis  is  formed,  the  gubernaculum  lies  behind  the  serous  sac.  The 
descent  of  the  testicle  is  accompanied  by  a  shortening  of  the  gubernacuiar  cord, 
which  thus  appears  to  draw  the  organ  downwards  into  the  scrotum,  and  the  testicle 
following  the  line  originally  taken  by  the  gubernacuiar  cord,  also  passes  down  along 
the  posterior  wall  of  the  processus  vaginalis,  which  it  therefore  invaginates  from 
behind. 

In  manv  animals  the  testicles  remain  throuofhout  life  in  the  abdominal  cavity.  In  others 
thev  only  descend  into  the  scrotum  dtiring"  the  period  of  "  heat."  Cases  of  cryptorchismus.  in 
■which  one  or  both  testicles  have  failed  to  reach  the  scrotum,  and  have  remarued  either  in  the 
ingTiinal  canal  or  -n-ithin  the  abdominal  cavity,  are  not  unfrequent  in  the  human  subject. 

The  ovaries  also  undergo  a  considerable  change  of  position,  accompanied  by  a 
shortening  of  the  band  which  corresponds  with  the  gubernaculum  testis  in  the  male. 
This  band,  as  it  passes  by  the  united  part  of  the  Mlillerian  ducts  which  are  forming 
the  body  of  the  uterus,  becomes  attached  laterally  to  that  organ,  and  the  descent  of 
the  ovary  is  normally  arrested  at  the  side  of  the  uterus.  In  rare  cases,  however,  the 
ovaries  pass  through  the  abdominal  ring  by  the  canal  of  Xuck,  and  may  even  Ije 
found  in  the  labia  majora,  where  they  resemble  in  position  the  testicles  within  the 
scrotum. 

The  External  Organs. — The  external  organs  are  up  to  a  certain  time  entirely 
of  the  same  form  in  both  sexes,  and  the  several  ors-ans  which  afterwards  distinguish 
the  male  and  female  externally  have  a  common  origin  (see  fig.  155).  A  cloaca  exists 
till  after  the  fifth  week,  and  the  genital  eminence  from  which  the  clitoris  or  penis  is 
formed  makes  its  appearance  in  the  course  of  the  fifth  or  sixth  week  in  front  of  and 
within  the  orifice  of  the  cloaca.  In  the  course  of  the  seventh  and  eighth  weeks  this 
orifice  is  seen  to  be  divided  into  two  parts  ;  but  the  exact  manner  in  which  the  sepa- 
ration of  the  two  apertures  takes  place  has  not  been  accurately  traced.     The  process 


12S 


THE  EXTERNAL  ORGANS. 


is  connected  with  the  formation  of  the  urogenital  cord  as  an  independent  structure, 
and  results  in  the  division  of  the  cloaca  into  a  dorsal  or  anal  and  a  ventral  or 
urogenital  part  {urogenital  sinus).  Somewhat  later,  in  the  ninth  or  tenth  week,  a 
transverse  integumental  band  completes  the  division,  which  band  forms  the  whole  of 
the  perineum  of  the  female,  and  the  part  of  the  perineal  integument  in  the  male 
which  is  situated  behind  the  scrotum. 

Of  the  two  apertures  the  dorsal  one  or  anus  is  of  small  size,  and  is  surrounded  by 
a  small  circular  integumental  ridge  ;    the  anterior  or  urogenital  aperture  forms  a 


Fig.   155. — Developjiext    of   the   external   sexual 

ORGANS      IN      THE     MALE     AND     FEJIALE     FK03I     THE 
INDIFFERENT    TYPE.      (Ecker. ) 

A,  the  external  sexual  organs  in  an  embryo  of  about 
nine  weeks,  in  which  external  sexual  distinction  is  not 
yet  established,  and  the  cloaca  still  exists  ;  B,  the  same 
in  an  embryo  somewhat  more  advanced,  and  in  which, 
without  marked  sexual  distinction,  the  anus  is  now 
separated  from  the  urogenital  aperture  ;  C,  the  same  in 
an  embryo  of  about  ten  weeks,  showing  the  female  type  ; 
D,  the  same  in  a  male  embi-yo  somewhat  more  advanced. 
Throughout  the  figures  the  following  indications  are 
employed  ;  ^c,  sexual  eminence  (penis  or  clitoris)  ;  to 
the  right  of  these  letters  in  A,  the  umbilical  cord  ;. 
'p,  penis  ;  c,  clitoris  ;  cl,  cloaca  ;  iig,  ui'ogenital  open- 
ing ;  a,  anus  ;  Is,  cutaneous  elevation  which  becomes 
labium  or  scrotum  ;  I,  labium  ;  s,  scrotum ;  co,  caudal 
or  coccygeal  elevation. 


nan-ow  vertical  slit  wider  behind  than  before,  and  running  forward  as  a  furrow  into 
the  rudim.ent  of  the  penis,  or  clitoris. 

The  well  marked  emmence  in  the  integument  which  forms  this  rudiment,  at  first 
indifferent  in  the  two  sexes,  is  surrounded  by  a  deep  circular  fold  of  the  integument 
which  encompasses  its  base,  and  which  is  the  foundation  of  the  mons  veneris  and 
labia  majora  in  the  female,  and  when  united  by  median  fusion,  of  the  scrotum  in  the 
male.  The  lips  of  the  urogenital  furrow,  which  in  the  female  are  converted  into  the 
nymphffi,  and  in  the  male  unite  as  the  integument  below  the  penis,  are  both  at  first 
precisely  the  same  in  all  embryoes.  In  the  open  condition,  which  continues  until 
the  eleventh  or  twelfth  week,  the  parts  appear  alike  in  both  sexes,  and  resemble 
the  more  advanced  female  organs.  The  rudiments  of  Bartholin's  or  Coivper's 
glands  appear  at  an  early  period  as  involutions  of  epitheKum,  near  the  root  of  the 
rudimentary  clitoris  or  penis,  on  each  side  of  the  genito-urinary  passage. 

In  the  female,  the  outer  circular  fold  of  integument  enlarges  at  the  sides  so  as  to 
cover  the  clitoris  as  the  labia  majora.  The  clitoris  itself  remains  relatively  small, 
and  the  groove  on  its  under  surface  becomes  less  and  less  marked,  owing  to  the 
opening  out,  and  subsequent  extension  backwards,  of  its  margins  to  form  the  nympha^. 
The  vascular  bulbs,  sunk  more  deeply  in  the  tissues  than  in  the  male  organ,  remain 
distinct  and  separate,  except  at  one  point  where  they  run  together  in  the  gians 
clitoridis.  The  hymen  begins  to  appear  about  the  fifth  month  as  a  fold  of  the  lining 
membrane  at  the  opening  of  the  genital  passage  into  the  urogenital  sinus.  Within 
the  vestibule,  which  is  the  shortened  but  widened  remains  of  the  urogenital  sinus, 
the  urethral  orifice  is  seen,  the  urethra  itself  undergoing  considerable  elongation. 

In  the  male,  on  the  contrary,  the  ^;e;^^s  continues  to  enlarge,  and  the  margins  of 
the  groove  along  its  under  surface  gradually  unite  fi'om  the  primitive  urethral  orifice 
behind,  as  far  forwards  as  the  glans,  so  as  to  complete  the  long  canal  of  the  male 
urethra,  which  is  therefore  a  prolongation  of  the  urogenital  sinus.  This  is  accom- 
plished about  the  fifteenth  week.     When  tlie  union  remains  incomplete,  the  abnormal 


THE    EXTERNAL   ORGAXS.  129 

condition  named  hypospadias  is  produced.  In  the  meantime  the  prepuce  is  formed, 
and,  moreover,  the  lateral  cutaneous  folds  also  unite  from  behind  forwards,  along  the 
middle  line  or  raphe,  and  thus  complete  the  scrotum,  into  which  the  testicles  descend 
in  the  course  of  the  eighth  month  of  foetal  life,  as  before  described. 

The  corpora  cavernosa,  which  are  at  first  separate,  become  united  in  their  distal 
portions  in  both  sexes  ;  but  the  corpus  spongiosum  urethrje  which  is  also  originallv 
divided  in  all  embryos,  and  in  the  female  remains  so  in  the  greater  part  of  its 
extent,  becomes  enlarged  in  the  male  in  the  glans  penis,  and  its  two  parts  become 
united  mesially  both  above  and  below  the  urethra,  so  as  to  enclose  the  whole  of  that 
tube  from  the  bulb  forwards  to  the  glans. 

The  following  Table  and  Diagrams  exhibit  the  correspouding  parts  of  the 
urino-generative  organs  in  the  two  sexes  : — 


180 


GENITO-UPJNARY    OI^GANS    OF    THE    TWO    SEXEa. 


V' 


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•asai^j  'lOfid  'iaoa:  'lono;  "SiiiNEig 

ivxmaf)      iiviaa'i'inpj;      ntm^io,/^     NYMLiiOj^^  T?-iiNa{)0'afi 


ILma-*- 


-l-H«.  V- 


1 


TlIK    KXTEiiNAL    UllGANS. 


131 


Fig.  156. — Diagrams  TO  snow  TiiK  develhi-mknt 

OK    MALE    AN1>    FEMALE    GENERATIVE    oUiiANS 

KuoM  A  COMMON  TYi'K.     (Allen  Thomsoii. ) 
A. — Diai;kam   w   the  pkimitive  uro-cjemtal 

liRiiANS  IN  the    embryo    PREVIOUS  TO  SEXVAL 
mSTINCTION. 

3,  ureter ;  4,  urinary  bladder  ;  5,  urachus  ; 
ot,  the  genital  ridge  from  which  either  the  ovary 
or  testicle  is  formed  ;  W,  left  Wolffian  body  ; 
w.w,  right  and  left  Wolffian  ducts  ;  m.m,  right 
and  left  ^lullerian  ducts  uniting  together  and 
ninuing  with  the  Wolffian  ducts  in  ijc,  the  genital 
cord  ;  luj,  .sinus  urogenitixlis  ;  /,  lower  jiart  of 
the  intestine  ;  cl,  cloaca  ;  cp.  elevation  which 
becomes  clitoris  or  penis  ;  h,  fold  of  integument 
from  which  the  labia  majora  or  scrotum  are 
formed. 


B. — Diagram  of  the  female  type 
OF  sexual  organs. 

o,  the  left  ovary  ;  po,  parovarium 
(epoophoron  of  Waldeyer) ;  W,  scat- 
tered remains  of  Wolffian  tubes  near 
it  (paroophoron  of  Waldeyer) ;  d  G. 
remains  of  the  left  Wolffian  duct,  such 
as  give  rise  to  the  duet  of  Gartner, 
represented  by  dotted  lines  ;  that  of 
the  right  side  is  marked  n:  ;  /,  the 
abdominal  opening  of  the  left  Fallo- 
jiian  tube  ;  u,  uterus  ;  the  Fallopian 
tube  of  the  right  side  is  marked  m  ; 
(J,  round  ligament,  corresponding  to 
gubernaculum  ;  i,  lower  part  of  the 
intestine  ;  va,  vagina  ;  h.  situation 
of  the  hymen  ;  C,  gland  of  Bartholin 
(Cowper's  gland),  and  immediately 
above  it  the  urethra  ;  cc,  corpus  ca- 
vernosum  clitoridis ;  sc,  vascular  bulb 
or  corpus  spongiosum  ;  n,  nympha  ; 
/,  labium  ;  v,  vulva. 


<;;. — Diagram  of  the  male  type  op 

SEXUAL    OPvGAXS. 

t.  testicle  in  the  place  of  its  original 
formation  ;  e,  caput  epididymis  ;  vd,  vas 
deferens  ;  W,  scattered  remains  of  the 
Wolffian  body,  constituting  the  organ  of 
(jiraldes,  or  the  paradidymis  of  Waldeyer; 
vh,  vas  aberrans  ;  m,  Miillerian  duct, 
the  upper  part  of  which  remains  as  the 
hydatid  of  Morgagni.  the  lover  part, 
represented  by  a  dotted  line  descending 
to  the  prostatic  vesicle,  constitutes  the 
occasionally  existing  coma  and  tube  of 
the  uterus  masculinus  ;  g,  the  guberna- 
culum ;  VH,  the  vesicula  seminalis  ;  pr. 
the  prost;tte  gland  ;  C,  Cowper's  gland  of 
one  side  ;  cp,  corpora  cavernosa  penis 
cut  short  :  gp,  corpus  spongiosum  ure- 
thrte  :  s,  scrotum  ;  t' ,  together  with  the 
dotted  lines  above,  indicates  the  direction 
in  ■which  the  testicle  and  epididymis  de- 
scend from  the  abdomen  into  the  scrotum. 


^t^^-^v^ 


132  EECENT    LITERATURE. 


RECENT    LITERATURE. 

Ackeren,  F.  v.,  Beitr.  sur  Entwickelunr/sgeschichte  d.  weihlichen  Sexualorgane  des  Menschen, 
Zeitschr.  f.  wiss.  ZooL,  1889. 

Beard,  J,,  The  origin  of  the  segmental  duct  in  Elasmohranchs,  Anat.  Anzeiger,  1887. 

Benda,  C,  Die  Entivicld.  des  Sdugethierhodens,  Verhandl.  der  anatom.  Gesellschaft,  1889. 

Bierfreund,  "M..,  Ue.  die  Einmundungsweise  der  Milller' schen  Gdnge  in  d.  Sinus  urogenitalis  hei 
dern  vienschliclien  Embryo,  Zeitsch.  f.  Geburtshlilfe  u.  Gynak,  xvii. 

Bonnet,  E..,  Ueber  die  ektodermale  Entstehung  des  Wolffs  schen  Ganges  iei  den  Sdugethieren, 
Miinchener  med.  Wocbensclir.,  1887  ;  Erabryologie  der  Wiederlcduer,  Arch.  f.  Anat.  u.  Physiol., 
Anat.  Abth.,  1889. 

Bramann,  F.,  Beitrag  zur  Lehre  von  dem  Descensus  testieulorum,  tfcc.  Arch.  f.  Anat.  u.  Physiol. , 
Anat.  Abth.,  1884. 

Brandt,  Ue.  d.  ZusammenJiang  der  Glandula  suprarenalis  rait  dem  Parovarium,  d-c,  Biol. 
Central bJ.  Ix. 

Cadiat,  0.,  M&moire  sur  l\d6rus  et  les  trompes  (developpement).  Joum.  de  ranatomie, 
1884  ;  Du  developpement  du  canal  de  I'urethre  et  des  organes  genitaux  de  Vembryon,  Journ.  de 
I'anatomie,  1884. 

Doh.rn,  Ueber  die  Gartner' schen  Kandle  heim  Weibe,  Arch.  f.  Gynakologie  ,  xxi.,  1883. 

Emery,  C. ,  Recherches  embryologiques  sur  le  rein  des  mammiferes,  Archives  italiennes  de  biologies 
t.  iv.,  1883. 

Flemmingr,  W.,  Die  eTctoblastische  Anlage  des  Urogenitalsy stems  beim,  Kaninchen,  Archiv.  f. 
Anat.  u.  Physiol.,  Anat.  Abth.,  1886. 

G-asser,  Embryonalreste  avi  indnnlichen  Genitalapparat,  Marburg.  Sitzungsb.,  1882. 

Gottsch.au,  M.,  StruJctur  u.  embryonale  EntwicH.  der  Nebennieren  hei  Sdugethieren,  Arch,  f, 
Anat.  u.  Physiol,  Anat.  Abth.,  1883. 

Haddon,  Suggestion  res2Jecting  the  epiblastic  origin  of  the  segmental  duct,  Proceed,  of  the  Royal 
Dublin  Society,  Feb.  16,  1887. 

Hoffmann,  C.  K.,  Zur  Entwicklungsgeschichte  der  Urogenitalorgane  bei  den  Anamnia,  Zeitschr. 
f.  wiss.  ZooL,  Bd.  xliv.,  1886  ;  Zur  Entwickl.  der  Urogenitalorgane  bei  den Bejytilien.  Zeitschr.  f.  wiss. 
Zool.  xlviii. 

Janosik,  I.,  Bemerhungen  iiber  die Enttvicklung  der  Nebenniere,  Archiv.  f.  mikrosk.  Anatomic,  Bd, 
xxii.,  18S3  ;  Histologisch-embryologische  Untersuchungen  ilber  das  Urogenital-system,  Sitzungsber.  d. 
Wiener  Akad.,  1885. 

Kallay,  A.,  Die  Niere  im  friihen  Stadium  des  Embryonallebens,  Mitth.  aus  d,  embryol.  Inst.  d. 
Univers.  Wien,  1885. 

Kocks,    Ueber  die  Gartner  schen  Gdnge  beim  Weibe,  Arch,  f.  Gynakologie,  xx.,  1883. 

Kollmann,  Ueber  die  Verbindung  zwischen  Colom  u.  Nephridium,  Festschrift.  Basel,  1882. 

Lock-wood,  C.  B.,  The  development  and  transition  of  the  testis,  normal  and  abnormal.  Journal  of 
Anat.  and  Phys. ,  vols.  xxi.  and  xxii.,  1887  and  1888. 

Martin,  E.,  Ueber  die  Anlage  der  Urniere  beim  Kaninchen,  Archiv.  f.  Anat.  u.  Physiol.,  Anat. 
Abth.,  1888. 

Martin,  P.,  Zur  Entwickl.  der  cavernosen  Korper,  <&c.,  Deutsche  Zeitschr.  f.  Thiermed.,  xvi. 

Milialkovics,  Gr.  v.,  Untersuchungen  iiber  die  Entwicklung  des  Ham-  und  Geschlechts-apparatof 
der  Amnioten,  Intern.  Monatsschr.  f.  Anat.  u.  Histol.,  1885,  1886. 

Mitsukuri,  K.,  The  ectoblastic  origin  of  the  Wolffian  duct  in  Chelonia,  Zool.  Anzeiger, 
1888. 

Nag-el,  Ueher  den  Wolff' schen  Korper  des  menschl.  Embryo,  Zeitschr.  f.  Geburtshulfe  u.  Gynak., 
1889  ;  Ueber  die  Entwickl.  des  Urogenitalsy  stems  des  Menschen,  Arch.  f.  mikr.  Anat.,  xxxiv.,  1889  ; 
Ue.  das  Vorkommen  von  Primordialeiern  ausserhalb  der  Keimdrilsenanlage  beim  Menschen, 
Anatomischer  Anzeiger,  iv. 

Perenyi,  J.  v.,  Die  ektoblastische  Anlage  des  Urogenitalsy  steins  bei  Eana  esculenta  und  Lacerta 
viridis,  Zoolog.  Anzeiger,  1887. 

Benson,  G-.,  Contributions  h  Vembryologie  des  organes  d'eoccritions  des  oiseaux  et  des  mammiferes. 
These.     Bruxelles,  1883. 

Both,  Ueber  einige  Urnierenreste  beim  Menschen,  Festschrift.  Basel,  1882. 

Btickert,  J.,  Ueber  die  Entstehung  der  Excretionsorgane  bei  Selachiern,  Arch.  f.  Anat.  u.  Physiol., 
Anat.  Abth.,  1888. 

Schmieg-elo-w,  E.,  Studien  ueber  die  Entwickl.  des  Hodens  u.  Nebenliodens,  Arch.  f.  Anat.  u. 
Physiol.,  Anat.  Abth.,  1882. 

Sedg-wick,  A. ,  On  the  early  development  of  the  anterior  part  of  the  Wolffian  duct  and  body  in  the 
c7«c^,  <£-c..  Quarterly  Journal  of  Microsc.  Science,  1881. 

Sem.on,  E,.,  Die  indifferente  Anlage  der  KeimdrUsen  beim  Hilhnchen  und  ihre  Differenzirung 
zum  Hoden,  Jenaische  Zeitschr.  f.  Naturw.,  Bd.  xxi.,  1887. 

Spee,  P.,  Ueher  directe  Betheiligung  des  Ektoderms  an  der  Bildung  der  Urnierenanlage  des 
Meerschweinchens,  Archiv  f.  Anat.  u.  Physiol.,  Anat.  Abth.,  1884;  Ueber  weitere  Befunde  zur  Ent- 
wicklung der  Urniere,  Mittheil.  f.  d.  Verein  Schleswig-Holstein,  1886. 

Strahl,  H.,  Zur  Bildung  der  Kloake  des  Kaninchenemhryo,  Archiv  f.  Anat.  u.  Phys.,  Anat.  Abth., 
1886  ;  Ueher  den  Wolff' schen  Gang  und  die  Segmentalbldschen  bei  Lacerta,  Marburger  Sitzungsberichte, 
1886. 


RECENT    LITERATURE.  133 

Tourneux,  R.  Sur  les premiers  developpements  du  doaque,  du  tvbercule  giniial  et  de  Vanuichez 
Vembruon  dTmouon,  Journal  de  lanatomie.  1888  ;  SvLr  U  divdoppemcnt  du  tubercle  gimtal  chez  le 
fZTf^rnainll-c.,  J^urn.  de  lanatomie.  xxv.  ;  Sur  le  mode  de  formation  du  j^imnie  chcz  lembryon 
tiu  mou^o/i,  Coiui.t.  rend,  de  la  Socidte  de  Biologic,  1890.  ,„.,.,  t^„^  j- 

Tourneux,  F.,  et  Legay,  Ch.,  Mdmoire  sur  U  ddvehppement  de  Cutirus  et  du  xwjxn,  Joum.  de 

''""weidon'^W.  F.  R.,  Note  on  the  ori'jin  of  the  suprarenal  bodies  of  vertebrates   Proceed    of  the 
aoyal  Society,   vol.  37,   1885  ;    On  the  suprarenal  bodies  of  vertebrata,  Quarterly  Journal  of  Micr. 

^"  Wieger.^a.,  Ueber  die  Entstehung  u.  Entwickl.  der  Bander  des  weiUichen  Genitalapparates,  «tc., 
\rch   f    Vnat.  u.  Phvsiol.,  Anat.  Abtb.,  1885.  ,       „       . 

Wijhe,  J.  W.  v.,  Die  Bethtiliyung   des  Ektoderms  an  der  Entwickluny  des   Vornurenganges, 

2ool.  Anz.,  ISSti. 


134 


DEVELOPMENT    OF   THE    HEART. 


FORMATIOlSr     OF    THE     VASCULAR    SYSTEM. 

DEVELOPMENT     OP     THE     HEART. 

In  mammals,  the  heart  appears  in  the  form  of  two  tnbes  lying  in  the  cephalic 
region,  one  on  either  side  of  the  embryo.  These  are  seen  at  a  very  early  period, 
prior,  in  fact,  to  the  separation  of  any  part  of  the  alimentary  canal  from  the  yolk 
sac,  and  to  the  closure  of  the  neural  groove.  This  bilateral  condition  was  first 
observed  by  Hensen  in  the  rabbit ;  it  has  been  seen  by  His  in  the  human 
embryo. 

The  situation  and  mode  of  formation  of  the  bi-tubular  heart  are  well  illustrated 
by  the  accompanying  figures  from  KoUiker.     They  exhibit  the  condition  in  the 


Fig.   157. — Rabbit  embeyo  of  the  9th  day,  feoji  the 
SURFACE.     -J-.     (Kolliker.) 

The  medullary  groove  is  enlarged  anteriorly  and  the  primaiy 
optic  vesicles  are  growing  out  from  the  first  cerebral  enlarge- 
ment. On  either  side  of  the  head,  the  bilateral  tubular  heart 
is  seen.     Eight  pairs  of  protovertebraj  are  formed. 

rabbit   embryo  of  about  eight  or  nine  days — the 
time  when  the   heart   first   makes  its  appearance. 
Fig.  157  shows  such  an  embryo  in  surface  view. 
The  neural  groove,  as  also  the  sections  show,  is 
widely  open,  although  the  rudiments  of  the  cere- 
bral eulargements  are  apparent  in  it,  and  also  the 
enlargements  for  the  primary  optic  vesicles.    There 
are   eight    pairs    of    protovertebras,   the   paraxial 
mesoblast  in  front  of  these  and  on  either  side  of 
the  cerebral  enlargements  being  undivided.     Out- 
side this  undivided  cephalic  mesoblast  is  a  short 
tube  dijiping  in  front  into  it,  and  passing  behind 
into   a   venous   trunk,   the   vitelline   or   omphalo- 
meseraic  vein   of   the   same   side.     The  tube   lies 
within  and  is  immediately  surrounded  by  a  clear 
space,  which  is  continued  forwards  beyond  it  on 
either  side   of  the   fore-brain  ;   this  space  is  pro- 
longed from  the  mesoblastic  cleft  or  pleuro-peri-/ 
toneal  cavity  (coelom). 
The  two  short  tubes  form  the  double  rudiment  of  the  heart.     The  situation 
which  they  occupy  becomes,  when  the  lateral  walls  fold  over  to  form  the  foregut, 
the  ventral  wall  of  the  pharynx,  and  the  two  tubes  are  thus  brought  together  in  the 
middle  line  underneath  the  head  part  of  the  ahmentary  canal.     Here  they  soon 
become  fused  together  to  form  a  single  median  tube,  the  hinder  end  of  which  is  still 
continuous  with  the  two  vitelline  veins,  while  the  anterior  end  bifurcates  near  the 
anterior  end  of  the  foregut  into  two  branches  which  arch  dorsalwards  on  either  side 
of  that  tube,  and  then"  pass  backwards  on  each  side  of  the  notochord  as  the  two 
primitive  aortee. 

These  changes  in  the  position  of  the  primitive  heart  are  partly  shown  in  surface 
view  in  figs.  158,  159,  but  they  can  only  properly  be  appreciated  by  the  study  of 
transverse  sections.  Fig.  160  is  a  transverse  section  through  the  anterior  head 
region  of  the  embryo  shown  in  fig.  157.  This  is  anterior  to  the  heart  region,  but 
shows  the  commencing  folding  over  of  the  splanchnopleure  to  form  the  foregut. 


DEVKLOl'MENT    OF    THE    HEART. 


135 


The  mesoblastic  cleft  (cceloin,  ;V/ )  is  somewhat  dilated,  but  is  not  doubled  in,  as  in  the 
heart  re<j;ion.  The  lateral  niesoblast  ceases  a  short  distance  beyond  it.  Fig.  IGl  is 
a  section  throiiirh  the  middle  of  the  head  region  of  the  same  embryo.     Here,  while 


/ 


C(\ 


Fin 


158. — EjIBRYO    rabbit    of    eight   days   AJTD    eighteen    hours,    with   9    PROTOTERTEBR.i:,    VIEWED 
FROJI    THE  VENTRAL    ASPECT.       "{.       (Kolliker. ) 


Fig.   159. — Sketches  showing  more  advanced  condition  of  the  bitubular  heart  of  the  rabbit. 

-f .  (Allen  Thomson. ) 
A,  view  from  below  of  an  embiyo  in  which  the  formation  of  the  heart  was  somewhat  more  advanced 
tlian  in  fig.  158,  and  of  which  an  outline  of  the  heart  is  repeated  in  B.  C,  from  another  embryo,  shows 
the  two  halves  of  the  heart  in  the  commencement  of  their  coalescence.  A,  the  part  of  the  bent  tube 
which  becomes  the  ventricle  ;  a,  primitive  aortic  arches  and  descending  aortte  ;  VV,  vitelline  veins 
entering  the  heart  posteriorly      The  arrows  indicate  the  course  of  the  blood. 

the  other  parts  of  the  section  are  much  the  same  as  in  front,  the  dilatation  of  the 
ccelom,  which  is  in  fact  the  rudiment  of  the  future  pericardium,  is  occupied  by  an 


^dd' 


dd      ^'    jih 


Fig.  160. — Section  from  the  same  embryo  further  forward  than  that  shown  in  the  following 

figure.     (Kolliker.) 

J),  paraxial  mesoblast ;  rf,  medullary  groove  ;  r,  ridge  bounding  groove  ;  mjp,  medullary  plate 
of  hind  brain  ;  h,  epiblast ;  Ji-p^  somatopleure ;  dfj>,  splanchnopleure  ;  ■ph,  anterior  part  of  ccelom  ; 
mes,  mesoblast  beyond  the  ccelom  ;  dd,  hypoblast ;  dd' ,  notochordal  thickening  ;  sv:,  lateral  wall  of  the 
developing  pharj-nx. 

invagination,  or  fold,  of  the  splanchnic  mesoblast.     This  fold  becomes  subsequently 
entirely  separated  and  the  aperture  or  line  of  invagination  closed ;  it  forms  the 


136 


DEVELOPMENT    OF    THE    HEAET. 


muscular  wall  of  the  heart.  It  encloses  a  second  tube  composed  of  flattened 
epithelium  cells  ;  this  so-called  endothelial  tuie  (His)  becomes  the  lining  epithelium 
of  the  endocardium. 

A 


IMS   fi 


iliJi  \f'^i^'' 


Fig.  161. — A,  Transverse  section  through   the  head   of  an  embryo  rabbit  op  eight  days  and 

FOURTEEN  HOURS,  WITH  A  PART  OP    THE    PERIPHERAL    BLASTODERM.         *f.       (KollikeV. ) 

lih,  rudiments  of  the  heart ;  &r,  pharyngeal  groove,  \vith  iiotochordal  thickening  of  hypoblast. 

B. — Part  of  the  same  more  highly  magnified,     ^'f-.     (Kolliker.) 

Lettering  as  in  fig.    160.      In  addition  : — alili,   fold  of   splanchnopleure  to  form  wall  of  heart ; 
ihli,  endothelial  tube  of  heart. 


There  is  some  doubt  as  to  the  source  of  this  endothelial  tube  of  the  heart.  In  the  preceding 
edition  of  this  work  it  was  stated  that  it  is  • '  derived  from  the  deeper  part  of  the  visceral  meso- 
blast  ;  "  this  statement  being  apparently  founded  upon  the  statements  and  figures  given  by 
Kolliker.  His  ascribes  it,  like  the  endothelium  of  the  blood-vessels,  to  an  ingrowth  from  the 
vascular  area.  The  appearance  of  the  section  shown  in  fig.  161  B,  seems  to  lend  colour  to  the 
belief  that  the  invagination  which  has  taken  jslace  to  form  the  heart  is  not  the  splanchnic 
mesoblast  only,  but  has  included  also  the  hypoblastic  layer  of  the  splanchnopleure  ;  the  notcli 
which  is  seen  in  the  hypoblast  near  .vx\  appearing  to  indicate  an  inteiTupted  connection  with 
the  endocardial  tube.  Should  future  investigations  show  that  this  is  actually  the  mode  of 
formation  of  the  tube,  the  mammalian  heart  would  be  developed  in  essentially  the  same 
manner  as  has  been  shown  by  Ktickert  to  occur  in  Pristiurus  (an  Elasmobranch),  where  this 
organ,  which,  as  in  all  vertebrates  below  mammals,  is  formed  only  after  the  foregut  is  com- 
IDleted,  is  developed  as  a  median  outgrowth  or  thickening  of  the  ventral  wall  of  the  foregut. 
A  similar  mode  of  formation  has  also  been  noticed  in  Cyclostomata,  Ganoids,  and  Amphibia.  In 
reptiles  and  birds  the  first  appearance  of  the  heart  is  as  a  bilateral  tube,  but  it  becomes  visible 
only  after  the  foregut  is  formed,  and  the  two  tubes  lie  from  the  first  close  together,  and  from 
the  surface  appear  as  a  single  median  tube. 

Sections  at  a  somewhat  later  period  (fig.  162)  show  the  two  tubes  lying  in  con- 
tact on  the  ventral  side  of  the  now  completed  foregut.  The  septum  which  divides 
them  at  this  period  has  nothing  whatever  to  do  with  the  permanent  intra-cardiac 
septum,  but  soon  becomes  completely  absorbed,  so  that  by  the  fusion  of  the  two 
lateral  tubes  a  single  median  tube  is  the  result  (fig.  163).  This  median  tube  remains 
attached  by  a  susp)ensory  membrane  resembling  a  mesentery  (rnesocardium  ])o&terms) 
{m}),  fig.  163)  to  the  ventral  Avail  of  the  pharynx,  but  the  rnesocardium  anterius, 
Avhich  also  at  first  results  from  the  fusion,  disap'iears,  except  at  the  lower  end',  and 
otherwise  the  tube  becomes  free,  except  where  the  vitelline  veins  pass  to  it  fi'om  the 
yolk  sac,  a  lateral  attachment  to  the  body  wall  being  here  subsequently  formed  on 
each  side  {rnesocardium  laterale  of  Kolliker).  After  it  is  thus  formed,  the  heart  is 
for  a  time  median  in  position  and  symmetrical  (fig.  164,  A),  but  already  in  the 


DEVELOPMENT    OF    THE    ilKAKT. 


137 


mammal  shows  distinct  imlications  of  division  into  its  seveml  parts  ;  indeed,  these 
parts  are  apparent  even  while  the  two  tubes  are  still  distinct,  as  the  accompanyin<? 
sketches  of  the  rabbit's  heart  clearly  show  (fig.  li)'J).     The  heart  does  not,  howevei" 


Fig.  lt>2. — Tkansvkusk  section  Tuuorcn 

THK  IIKOIOX  1>K  THK  HKAKT  IX  A  KAltlllT 
KMUKYo  I'F  MSK  HAYS.  SHOWIMi  THK 
COMMEXCIXi:  KISION  OF  TUK  TWO  TlUE:!. 

■y.     (Kolliker.) 

jj,  jugular  veins ;  ao,  desccinling  aortic  ; 
pit,  pliarynx  ;  hp,  cpiblast  of  loily-wall ; 
ih,  endothelial  lining  of  the  still  iliviileil 
heart :  a/i,  outer  wall  irt"  the  heart ;  ji,  jieri- 
eanlial  c.iloin  ;  df,  df,  visceral  niesoljlast 
(soniatopleurc) ;  e',  prolongation  of  the 
liyjiol'last  of  the  foregut  and  the  anterior 
will  of  the  pericardial  cavity  into  the 
l>artition  hetween  the  two  halves  of  the 
heart;  W,  bilaminar portion  of  blastoderm 
forming  pro-aninion  ;  I'ct,  ent,  its  two 
layers  (epiblast  and  hypoblast). 


a  It    in 


long  retain  its  symmetrical  position.  It  soon  becomes  bent  upon  itself,  so  as  to 
assume  the  shape  of  an  8,  the  anterior  part  of  the  tube  bending  over  to  the  right  and 
the  posterior  to  the  left  (tig.  164,  B).    At  the  same  time  the  posterior,  or  sino-auricular 


Fig.  163. — Section  THBorcH  the  segion  of  the  heart  in  a  rabbit  embryo  of  10  days,  after 

THE   TWO    TUBES    HAVE    UNITED    IXTO  A  SINGLE   MEDIAN    ORGAN,      (KoUikcr. ) 

rto,  descending  aoiiie  ;  ha,  bulbns  aortre  ;  ah,  its  external  wall  ;  mp,  posterior  mesocardiiim,  uniting 
the  heart  to  the  ventral  wall  of  the  pharynx,  ph,  and  here  separating  the  ijleuropericardial  ccelcm, 
p.  into  two  halves,  which  are,  however,  united  on  the  ventral  side  of  the  heaii;  ;  cut,  hyiioblast  of  yolk 
sac  ;  df,  its  mesoblast ;  df ,  mesoblast  of  pharynx  ;  ect,  epiblast. 

Fig.  164. — Outlines  of  the  anterior  half  of  the  embryo  chick  viewed  from  below,  shewing  the 

HEART  IN  ITS  EARLIER  STAGES  OF  FORMATION.        (After  Kcmak.  )       -^^ 

A,  embryo  of  about  28  to  30  honi-s  ;  B.  of  about  36  to  40  hours  ;  «,  anterior  cerebral  vesicle  ; 
h,  protovertebral  segments  ;  c,  cephalic  fold  ;  1,  1,  vitelline  or  omphalo-mesenteric  veins  entering  the 
heart  jjosteriorly  :  '2.  their  union  in  the  posterior  part  of  the  heart  ;  3,  the  middle  part  of  the  tnle 
coiTesponding  to  the  ventricle  ;  4  (in  B)  the  arterial  bulb. 

end  of  the  heart,  gradually  comes  to  lie  behind  or  dorsal  to  the  ventricular  part, 
which  arches  transversely  from  left  to  right,  -where  it  turns  shanply  upward  (towards 
the  head),  and  terminates  in  the  bulb.  The  tube  is  divided  by  slight  constrictions 
into  successive  portions,  viz.  :  (1)  the  part  formed  by  the  jimction  of  the  principal 
veins,  sinus  venosus  ;  (2)  the  auricular  part  ;  (3)  the  ventricular  part ;  and  (4)  the 
aortic  bulb. 


138 


DEVELOPMENT  OF  THE  HEAET. 


The  sinus  venosus  may  be  described  as  consisting  of  two  lateral  enlargements  or 
horns,  and  of  a  transverse  part  connecting  these  horns.  The  veins  which  it  at  this 
time  receives  are   the   nmbilical,  the  vitelline,  and  the  ducts  of  Cuvier  (formed 


a.    OM.  '1(1.1). 


Fig.  165. — Condition  of  the  heakt  in  the  human  embkto  of  about  fifteen  days,  reconstkucted 

FROM  serial  sections.     (His.)     *^ 

A,  from  before,  showing  external  appearance  of  heart ;  B,  the  same  with  the  muscular  substance  of 
heart  removed  showing  the  endothelial  tube  ;  C.  from  behind. 

mil,  mandibular  arch  with  maxillary  process;  liy,  hyoidean  arch  ;  6.  a,  bulbus  aortse ;  v,  right 
ventricle  ;  v' ,  left  ventricle  ;  au,  auricular  part  of  heart  ;  c.a.,  canalis  auricularis  ;  s.r,  horn  of  sinus 
venosus  with  umbilical  vein  {u.v),  superior  vena  cava  {v.c.s),  and  vitelline  vein  entering  it  ; 
d,  diaphragm  ;  m  j),  mesocardium  posteriiis  ;  I,  liver  ;  h.d,  bile  duct. 

by  the  junction  of  the  primitive  jugular  from  the  head  and  the  cardinal   from 
the  trunk).     The  three  veins  are  nearly  symmetrical  on  the  two  sides,  and  enter  the 


Fig.   166. — Heart  OP  A  somewhat  MORE  advanced  HUMAN   EMBRYO.       (His.)      \" 

A,  from  befoi-e  ;  B,  from  behind. 

r.v,  right  ventricle  ;  l.v,  left  ventricle  ;  h.a,  bulbus  aortse  ;  r.au,  right  auricle  ;  l.au,  left  auricle  ; 
v.c.s,  vena  cava  superior  ;  -w.r,  umbilical  vein  ;  v.v,  vitelline  vein  ;  d,  diaphragm. 

corresponding  horn  of  the  sinus  (fig.  168).  The  sinus  is  at  first  in  free  communi- 
cation with  the  common  auricular  cavity,  but  the  junction  presently  becomes 
narrowed,  and  the  resulting  aperture,  which  eventually  acquires  a  slit-like  character,, 


DEVELOPMENT    OF    THE    IIKAUT. 


1:39 


is  fdimd  to  oix'ii  rroiu  the  ri^iit  linni  of  the  sinus  into  tho  right  part  <ii"  the  commou 
auticle.  Tlie  sinus  now  forms  a  transversely  disposed  sac,  lying  below  and  beiiind 
the  common  auricle,  with  a  larger  right  and  a  smaller  left  liorn  (the  latter  being 


II 


Lull. 


f.aii. 


-^h.,1. 


Fig.  Id". — Hkart  of  noMAN  kmbryo  slightly  moue  advanced  than  that  shown  in  fig.  166.     (His.) 

A,  interior  of  auricle  aiul  ventricle  displayed. 

B,  endothelial  tube. 

«.c,  auriculai'  canal  ;  a.i,  area  interposita  of  His  ;  m,  posterior  mesocai'dium  ;  r.av,  l.au,  right  and 
left  auricles  ;  l.v,  left  ventricle  ;  r.r,  right  ventricle  ;  h.a,  lailbus  aorta-. 

ta])ered  off  into  the  left  duct  of  Cuvier) ;  in  this  condition  it  has  been  termed  by  His 
sacms  rciiniens  (fig.  169,  B,  and  fig.  171).  The  umbilical  and  vitelline  veins  soon  open 
into  it  by  a  common  trimk,  which  becomes  the  upper  end  of  the  vena  cava  inferior. 


oco' 


V.C.S.-~,^--S 


Fig.  168. — Heakt  OF  rabbit  EMBRYO.     (Lioru. ; 

A,  from  before ;  B,  from  behind.  ^  . 

s.s,  sinus  venosus  ;  l.r,  left  ventricle  ;  r.r,  right  ventricle  ;  b,  bulbus  aorta; ;  ao  ,  first  aortic  arch  ; 
ao",  second  aortic  arch  ;  r.au,  right  auricle  ;  l.au,  left  auricle  ;  itmb.v,  umbilical  vein  ;  vi.v,  vitelline 
vein  ;  v.c.s,  vena  cava  superior;  me,  posterior  mesocardiura. 

The  slit-like  orifice  of  the  sinus  in  the  back  of  the  right  auricle  is  guarded  by  two 
valve-like  folds  of  the  endocardium,  which  project  into  the  cavity  of  the  auricle  (right 


140 


DEVELOPMENT    OF    THE    HEART, 


LS.r. 


vunh.  v.Uvi.v.       c/^.v. 


Fie.  169. — A^'TEKioR  and  posterior  aspect  of  the  heart  of  a  so^fewhat  older  rabbit  embryo. 

vBorn.) 

p.a,  pulmonai-y  arteiy  ;  s.r,  s.l,  s.tr,  right  and  left  horns  and  transverse  part  of  sinus  respectively; 
r.s.c,  l.s.c,  right  and  left  superior  cavse  :  ^je,  apertuj-e  of  pulmonary  vein;  h.v,  hepatic  veins;  d.v, 
ductus  venosus  ;  me,  mesocardium  posterius. 

The  other  letters  as  in  iis.  16S. 


oci^ 


.;K'kT^l£?^  '^-^• 


Fiw.  170. — Section  through  the  heart  of  a  rabbit  EiiBRTO  at  the  stage  sno'wy  ix  fig.  169. 

(Born.) 

r.s,  l.s,  right  and  left  horns  of  sinus  receiving  from  above  the  respective  sujierior  vens  cava  :  r.au, 
l.au,  right  and  left  auricles  ;  r.v,  l.v,  right  and  left  parts  of  the  ventricle  ;  r.v.v,  l.f.v,  right  and  left 
valves  guarding  the  orifice  from  the  right  horn  of  the  sinus  into  the  right  auricle;  au.v.c.  one  of  the 
t>vo  endocardial  cushions  which  are  beginning  to  subdivide  the  common  amiculo-ventricular  aperture. 
The  dotted  line  encloses  the  extent  of  the  endocardial  thickening  ;  s',  first  septum  superior  growing 
down  between  ths  auricles  and  prolonged  below  by  a  thickening  of  endocardium.  Close  to  this  septum 
in  the  left  auricle  is  seen  the  opening  of  the  pulmonary  vein  ;  s.  inf,  inferior  septum  of  the  ventricles. 

Fig.  171. — View  FROii  behind  of  the  heart  of  a  hfjian  embryo  of  about  4  weeks,  magnified.  (His.) 

The  two  sux^erior  cavje,  right  and  left,  and  the  inferior  cava  are  seen  opening  separately  into  the 
sinus  which  is  a  transversely  elongated  sac  communicating  only  by  a  narrow  orifice  with  the  right 
auricle. 


DEVELOPMENT    UF   THE    HEART. 


141 


and  left  venous  valves)  (fi<?.  170,  r.v.v.,  Lv.v.).  These  pass  above  into  a  muscular  fold 
of  the  auricular  wall,  whicli  extends  over  the  roof  of  the  auricle  heart  parallel  to  the 
septum  atriurum,  and  is  known  as  the  septum  spHrimn  (Hi,'.  1 7o,  B).^  It  disajipears  at 
length,  probably  by  uniting  with  the  septum  atriorum.  Subse<|uently  the  venous  orifice 
opens  out,  and  the  right  horn  of  the  sinus,  which  is  now  seen  to  receive  all  the  great 
veins  except  the  left  duct  of  Cuviei',  becomes  gradually  incorporated  with  the  cavity 
of  the  auricle.  The  transverse  part  of  the  sinus  and  its  left,  horn  are  continuous 
with  the  left  duct  of  Cuvier  (tig.  171),  and  eventually  tiie  transvei-se  part  Ibrins  the 
coronary  sinus.  From  the  right  venous  valve  the  Eustachian  valve  is  formed,  and 
t'.ie  development  of  the  Thebesian  valve  is  also  connected  with  its  lower  end 
(Schmidt).     The  left  venous  valve  distippears. 

The  trausN'crsely  placed  \'eutricular  part  of  the  heart  receives  at  first  at  its  left 
end  the  orifice  of  the  common  auricle,  which  opens   iuto  its  posterior  wall  (fig. 


f.cuo.-v:^ 


S.ir^J. 


S.T.-n.f 


Fis.  i: 


-Diagram  to  show  the  formation  of  the  septum  of  the  ventricles  and  bi'lb,  and  the 

MODE  of  division  OP  THE  COMMON  AURICULO-VENTRICULAR  APERTURE.       (Born.  ) 

au.v.c  (in  A  and  B),  auriculo-ventricular  aperture,  partially  divided  into  two  by  endocanlial  cushions  ; 
r.au.v,  l.au.w  right  and  left  auriculo-ventricular  apertures  which  have  resulted  from  the  division  of  the 
common  aperture;  ;•. r,  l.v,  right  and  left  ventricles  ;  6,  bulbus  aortae,  replaced  in  C,  by  i^.a  and  a.o, 
pulmonary  artery  aud  aorta  ;  s.h,  septum  bulbi  ;  s.  ('«/,  septum  inferius  ventriculorum  ;  o  (in  A),  orifice 
between  the  two  ventricles. 

172,  A,  a.v.c.).  At  its  right  end  it  turns  sharply  upwards  into  the  aortic  bulb,  into 
which  it  gradually  tapers,  although  there  is  at  a  certain  point  a  constriction  of  the 
endothelial  tube,  where  the  semilunar  valves  are  subsequently  formed  {freium  Halleri). 
Soon  the  right  aud  left  halves  of  the  ventricle  are  separated  externally  by  a 
groove  which  extends  from  below,  partially  encircling  the  tube  (fig.  1G9).  If  the 
interior  of  ihe  heart  is  examined  at  this  stage,  it  is  seen  that  a  muscular  septum, 
corresponding  internally  to  this  groove,  is  growing  upwards  and  backwards  fi'om 
the  antero-inferior  part  of  the  tube,  and  is  gradually  separating  it  into  two  parts, 
which  become  the  right  and  left  ventricles  respectively  (fig.  170,  s.inf).  This 
iseptum  {septum  inferius  of  His)  is  placed  obliquely  to  the  long  axis  of  the  tube, 
and  extends  eventually  nearly  to  the  level  of  the  auriculo-ventricular  orifice,  which 
has  by  this  time  become  shifted  along  the  posterior  wall  of  the  tube,  so  as  to 
open  into  it  about  its  middle  instead  of  at  the  left  end,  as  was  previously  the  case 
(ng.  172,  B).  The  septum  of  the  ventricles  remains  incomplete  for  some  time, 
a  communication  between  the  two  ventricles  being  maintained  above  it.  Even- 
tually the  septum  inferius  unites  with  prolongations,  (1)  from  the  endocardial 
cushions  which  divide  the  common  auriculo-ventricular  orifice  into  right  and  left 


^  This  muscular  prolongation  maj',  as  Born  suggests,  be  of  use  in  assisting  the  action  of  the  valves, 
and  in  preventing  their  being  forced  backwards  into  the  sinus  when  the  auiicle  contracts. 


143  DEVELOPMENT  OF  THE  HEART. 

orifices  ;  (2)  from  the  endocardial  aortic  septum,  which  divides  the  bulb  into  aorta 
and  pulmonary  artery.  Thus  the  septum  of  the  ventricles  is  completed  by  endo- 
cardial connective  tissue,  a  fact  which  is  indicated  even  in  the  adult  heart  by  the 
existence  of  the  thin  septum  membranaceum  which  forms  the  uppermost  part  of 
the  interventricular  septum. 

The  common  auricle  in  the  meantime  becomes  shifted  relatively  upwards  over 
the  back  of  the  ventricles,  carrying  the  sinus  along  with  it,  but  it  still  lies  behind 
rather  than  over  the  ventricles,  and  the  aperture  of  communication  passes  from 
behind  forwards,  from  the  left  part  of  the  auricle  into  the  corresponding  half  of  the 
ventricle.  This  constricted  aperture  soon  becomes  elongated  into  a  short  canal, 
which  is  known  as  the  auricular  canal.  Its  orifice  into  the  ventricle  is  from  the 
first  somewhat  flattened,  and  bounded  by  two  lips,  an  upper  and  a  lower.  As  deve- 
lopment proceeds,  it  broadens  out  towards  the  median  plane  of  the  ventricular  tube, 
and  becomes  gradually  shifted,  first  towards,  and  eventually  over  the  line  of  constric- 
tion which  marks  off  the  future  right  and  left  ventricles  from  one  another  (fig.  172). 
The  ventricular  septum  has  by  this  time  extended  almost  up  to  the  transversely 
elongated  slit-like  orifice,  and  its  lips,  still  upper  and  lower  in  relative  position, 
become  greatly  thickened  by  the  formation  of  cushions  of  endocardium,  which  grow 
towards  one  another  in  the  middle  of  the  slit,  and  presently  fuse  into  a  median 
thickening  which  converts  the  single  I— l-shaped  aperture  into  two  triangular 
openings,  leading  one  into  each  ventricle  (fig.  172,  C).  Meanwhile,  the  septum  of 
the  ventricles  growing  towards  the  base  abuts  against,  and  at  length  comes  into  direct 
continuity  with  the  fused  endocardial  cushions,  but  this  connection  is  nearer  to  the 
right  than  to  the  left  auriculo-ventricular  aperture.^  There  is  still,  as  above  stated, 
a  small  orifice  of  direct  communication  between  the  left  and  right  ventricles  above 
the  free  edge  of  the  ventricular  septum,  and  this  is  not  closed  until  the  descent  of  the 
septum  of  the  bulb,  and  its  union  with  the  septum  of  the  ventricles,  completes  the 
interventricular  septum. 

The  above  account  of  the  division  of  the  auricular  canal  is  based  upon  that  given 
by  Born  for  the  rabbit,  and  in  some  respects  differs  from  the  description  which  was 
given  by  His  from  an  examination  of  human  embryos.  According  to  His,  the  endo- 
cardial cushions,  which  by  their  union  subdivide  the  auricular  canal,  are  preceded  by 
and  connected  with  a  growth  of  endocardial  tissue,  which  springs  from  the  posterior 
auricular  wall,  and  they  together  form  a  septal  prolongation  {septum  intermedium), 
which  projects  like  a  stopper  into  the  auricular  canal,  and  divides  the  latter  into  the 
two  auriculo-ventricular  orifices,  and  also  grows  down  beyond  that  canal  to  meet  the 
uprising  ventricular  septum  (fig.  173).  The  shortening  of  this  canal  is  in  part  effected 
by  a  kind  of  intussusception  which  takes  place,  and  which  causes  its  wall  to  be 
folded  into  the  ventricular  cavity ;  these  folds,  with  probably  some  thickening  of 
endocardium,  form  the  bases  of  the  lateral  flaps  of  the  auriculo-ventricular  valves 
(fig.  175).  The  bases  of  the  mesial  or  septal  flaps  are  formed  by  a  downward  growth 
of  the  edges  of  the  endocardial  septum  between  the  two  orifices.  Both  lateral  and 
mesial  flaps  become  continuous  Avith  the  spongy  muscular  substance  which  at  this 
time  occupies  most  of  the  cavity  of  the  ventricles  (fig.  175).  As  development 
proceeds,  the  flaps,  which  are  at  first  thick  and  soft,  become  thin  and  membranous, 
and  become  free  fi'om  muscular  substance  except  near  their  free  edges.  These 
muscular  bands  become  tendinous  near  their  insertion  into  the  valves,  and  thus  form 
the  chordcB  tendinea, ;  the  parts  which  are  not  thus  transformed  become  the  impillary 
muscles. 

The  septum  of  the  auricles  appears  at  the  upper  and  back  part  of  the  auricular 

2  Hence  the  riglit  auriculo-ventricular  orifice  lies  close  to  the  ventricular  septum,  but  the  left  orifice 
is  separated  from  it  by  an  interval,  into  which  the  root  of  the  aorta  becomes  continued  (Born). 


DEVELOPMENT    OF    THE    HEAHT. 


143 


cavity,  where  its  situation  is  externally  marked  by  a  j^roovc.  The  free  eds^e  of  this 
septum  throws  forwards  and  downwards,  and  the  septum  (W<f.  17<»,  .s')  j;radually 
separates  the  aiii'icular  cavity  into  a  right  and  left  half,  the  separation  being  com- 
pleted l)y  the  junction  of  its  free  edge,  which  shows  a  distinct  endocardial  thickening, 


/-«" 


s.sp.     s.s. 


l.cu 


Fig.    173. — Two  STAGES  IX  the  FOKIIATIOX  OF  THE   SEPTUM  I^•TERMEDI^^I  IX    THE    HEART    OF    THE    HCMAX 

EMBEYO.       (His.) 

In  A  the  septum  is  represented  as  growing  from  a  triangular  area  to  the  left  of  tte  sino-auricular 
orifice  ;  in  B  it  has  coalesced  with  the  endocardial  cushions,  and  lies  like  a  stopper  in  the  auricular 
■  canal. 

r.a,  La,  right  and  left  auricle ;  r.v,  l.v,  right  and  left  ventricle;  s.r,  sinus  venosus;  Eu.v,  Eustachian 
valve  ;  s.sp,  septum  spuriura  ;  s.s,  septum  superius  ;  s.inf,  septum  inferius  ;  s.i,  septum  intermedium  ; 
v.c.s,  vena  cava  superior  dextra. 


144  DEVELOPMENT    OF   THE    HEART. 

with  the  fused  cushion-like  thickenings  which  are  subdividing  the  common  auricular 
orifice.  But  before  the  originally  free  communication  between  the  two  auricles  is  thus 
closed,  a  new  aperture  makes  its  appearance  above  and  at  the  back  of  this  septum, 
and  gradually  enlarges,  so  that  a  passage  is  thus  re-established,  but  in  a  different 
situation.  This  new  orifice  is  the  foramen  ovale.  It  becomes  closed  by  a  second 
septum,  which  also  starts  from  the  superior  auricular  wall,  a  little  to  the  right  of  the 
original  attachment  of  the  first  septum,  and  gradually  grows  forwards  and  downwards 
over  the  orifice.  This  second  septum  becomes  the  limdus  Vieussenii,  the  first  one 
forms  the  so-called  valve  of  the  foramen  ovale  (Born). 

According  to  His'  account  of  the  process  in  the  human  embryo,  the  septum  atriorum  is 
formed  by  an  anterior,  or  lower,  and  a  posterior,  or  upper,  sickle-shaped  projection,  which  between 
them  enclose  the  foramen  ovale,  and  form  respectively  the  limbus  Vieussenii  and  the  valve  of 
the  foramen  ovale ;  the  connective  tissue  growth  which  he  describes  as  growing-  from  the 
posterior  aiiricular  wall  towards  the  auriculo-ventricular  orifice  takes  an  important  part  in 
the  formation  of  the  lower  septal  projection  (septum  intermedium).  There  is  reason,  how- 
ever, to  believe  that  the  process,  as  above  described  by  Born  for  the  rabbit,  is  materially  the 
same  in  all  the  higher  vertebrates,  including  man,  and  that  the  successive  growth  of  both 
septa  from  the  upper  and  posterior  auricular  wall  was  not  noticed  by  His  on  account  of  the 
lack  of  a  series  of  human  embryos  sufficiently  complete  to  show  all  the  stages  of  growth. 

Somewhat  late  in  the  course  of  development  (after  the  appearance  of  the  auricular 
septum),  the  pulmonary  veins  are  seen  entering  the  left  auricle.  Before  reaching  the 
auricle  they  have  united  to  form  a  single  vessel,  and  this  opens  into  the  auricle  near 
the  septum  (fig.  170,  p.v.).  In  some  animals,  as  the  rabbit,  this  represents  the 
permanent  mode  of  termination  of  the  pulmonary  veins,  but  in  man  the  right  and 
left  veins  come  to  open  separately  into  the  auricular  cavity,  either  by  division  of  the 
common  trunk  (His),  or  by  opening  out  of  the  common  trunk,  and  its  absorption 
into  the  auricle  in  the  same  way  as  the  right  horn  of  the  venous  sinus  is  absorbed 
into  the  right  auricle  (Born).  The  two  resulting  vessels  may  again  divide,  so  that 
four  pulmonary  veins  ultimately  terminate  in  the  left  auricle. 

The  aortic  bulb  becomes  subdivided  into  two  vessels,  the  ascending  aorta  and 
the  pulmonary  artery.  The  division  is  produced  by  a  septum  which  arises  as  two 
longitudinal  thickenings  of  the  lining  membrane  (endocardium).  These  grow  from 
opposite  sides,  and  gradually  meeting,  fuse  together  in  the  middle  of  the  bulb.  The 
folds  take  an  oblique  course  down  the  bulb,  for  above  they  are  anterior  and  posterior, 
but  below  are  right  and  left,  hence  the  resulting  vessels  after  separation  are  anterior 
and  posterior  below  and  right  and  left  above.  The  endocardial  thickenings  extend 
somewhat  below  the  origin  of  the  bulb,  and  unite  with  one  another  and  with  the 
septum  of  the  ventricles,  which  they  complete,  and  of  which  they  form  the  mem- 
branous part.  The  ventricular  part  of  the  heart  is  now  completely  divided  into  two, 
each  communicating  with  the  corresponding  division  of  the  arterial  bulb.  There  are 
at  first  no  semilunar  valves,  the  soft  thickened  endocardial  tissue  of  the  bulb  appear- 
ing to  exercise  a  sort  of  valvular  action.  The  valves  are  formed  as  three  projec- 
tions of  this  tissue  at  the  base  of  each  vessel,  at  first  thick  and  soft,  but  subsequently 
becoming  thinner  and  membranous.  The  common  aortic  trunk  has  four  such 
thickenings  at  the  lower  end,  and  the  septum  of  the  bulb  as  it  descends  is  prolonged 
into  the  right  and  left  of  these,  so  that  the  dumb-bell-shaped  orifice  is  divided  into 
two  triangular  apertures,  the  bulging  sides  of  which  are  formed  by  the  endocardial 
cushions  and  become  developed  into  the  semilunar  valves  (fig.  174). 

The  aortic  septum  begins  between  the  fourth  and  fifth  aortic  arches,  and  is  so  dis- 
posed, that  the  fourth  arch  continues  the  aortic  half  of  the  bulb,  the  fifth  the  pul- 
monary half.  After  the  completion  of  the  septum,  an  external  groove  makes  its 
appearance  along  the  line  of  the  endocardial  thickenings,  and  deepening  gradually, 
splits  the  bulb  into  two  separate  vessels. 


DEVELOPMENT   OF    THE    HEART. 


U5 


Fig.  17t.  —  Pl.VIJUAM  SHI)\VIN<i  TlIK 
DIVISION  OK  TUK  LOWKK  TAUT  OK 
THK  IIUI.IU'S  AOKT.*;,  AND  THK  KOK- 
MATIOM  <IK  THK  SKMIl.LNAK  VAI.VKS. 

(After  (rogciibiiur  ami  His.) 

A,  iindividctl  truncii.^  arteriosus 
with  fo\ir  (Miiiocariliai  cusiiions;  1!, 
advance  of  the  two  lateiiil  cusiiidiis 
resulting  in  the  division  of  tiie  liinifii ; 
C,  projection  of  tiiroe  endocardi.il 
cushions  in  each  juirl  ;  D,  the  sejiara- 
tion  into  aorta  and  imUnonary  trunks 
completed. 


Fig.   IT'i. — Sections    Tiinoi'iiii    Tin: 

HKAKT  OK  lU'.MAN  EMHKVuS,  SHoW- 
IXt!  TWO  STAdES  IX  THE  FORMATION 
OK  THE  CAUniAC  SEPTA  AND  OK  THE 
AURICULO-VENTKICULAK    VALVES. 

(His.) 

A,  from  an  embryo  of  n  or  6  week.s. 
Ji.  V,  riglit  auricle;  L.  V,  left  auri- 
cle ;  S.r.il,  right  horn  of  sinus  ;  S.r.)<, 
left  horn  of  sinus  ;  V.A\  Eustachian 
valve  ;  .i.int,  septum  superior  and 
endocardial  cushion  (septum  intei'me- 
diuni,  His)  ;  s.inf,  septum  infcrius 
ventriculorum.  This  septum,  as  well 
as  the  bulk  of  the  ventricle,  is  a  mu.s- 
cular  sponge  at  this  stage.  Oe,  oeso- 
phagus ;  JJi;  bronchus. 

B,  from  a  somewhat  more  advanced 
embryo.  Ad,  As,  right  and  left 
auricle ;  Ost,  auriculo-ventricular 
apertures ;  S.s.  septum  superior  of 
auricles  ;  S.!t,  endocai'dial  cushion 
(septum  intermedium)  ;  S.if,  septum 
inferius  ventriculorum,  now  denser 
and  more  muscular  ;  ^^pi^,  peri- 
cardial attachment. 

Distinct  muscular  tissue  is 
seen  in  the  cardiac  wall,  even 
as  early  as  the  stage  of  an 
S-shaped  tube,  although  the 
heart  begins  to  pulsate  regu- 
larly long  before  this.  The 
muscular  layer  is  separated 
from-  the  epithelial  lining  of 
the  cavities  (endothelial  tube 
of  His)  by  a  layer  of  clear 
gelatinous  tissue,bridged  across 
by  fine  fibres  (embryonic  con- 
nective tissue).  This  layer  is 
most  abundant  in  the  ventri- 
cular part  and  aortic  bulb,  and 
here  the  endothelial  tube  is 
consequently  much  smaller 
than  the  muscular  tube.  Sub- 
sequently, in  the  ventricle,  the 
gelatinous  tissue  is  invaded  by 
muscular  bands  which    grow 


146 


DEVELOPiMENT    OF    THE    PRINCIPAL    ARTERIES. 


into  it  from  the  compact  outer  layer  of  muscle,  and  unite  with  one  another  to  form 
a  spongework  of  muscular  trabeculse,  while  the  endothelium  of  the  cavity  becomes 
depressed  between  and  over  these  trabecuisB,  and  lines  all  the  spaces  between  them, 
which  thus  communicate  with  the  cavity  of  the  ventricle.  The  ventricles  are  therefore 
now  in  the  same  condition  in  Avhich  they  are  found  permanently  in  many  of  the 
lower  vertebrates  (e.g.  frog). 

Ultimately  the  compact  outer  layer  of  muscle  becomes  greatly  increased  in  thick- 
ness, and  the  spongework  of  trabeculge  occupies  a  relatively  much  smaller  portion  of 
the  cavity,  being  developed  in  part  into  the  columnge  carnese  of  the  adult  heart. 

Peculiarities  of  the  foetal  heart. — Besides  the  peculiarities  of  structure, 
which  have  been  above  descril)ed,  tlie  IVjetal  heart  differs  in  position,  in  relative  size, 
and  in  the  thickness  of  its  several  pares,  from  the  organ  after  birth.  Thus  it  is  at 
first  placed  immediately  under  the  head,  but  subsequently,  with  the  development  of 
the  neck,  it  gradually  assumes  a  position  farther  back.  In  early  foetal  life  it  is 
much  larger  in  proportion  to  the  size  of  the  body  than  at  a  later  period,  and  at  birth 
it  is  still  proportionally  large.  The  walls  of  both  ventricles  are  of  equal  thickness 
during  foetal  life,  a  peculiarity  which  is  evidently  connected  with  the  fact  that  in 
consequence  of  the  communication  of  the  pulmonary  artery,  through  the  ductus 
arteriosus,  with  the  aorta,  the  blood  pi'essure  which  they  have  to  overcome  is  the 
same. 

DEVELOPMENT    OP    THE    PPaNCIPAL    AKTERIES. 

From  the  point  of  insertion  of  the  aortic  bulb  into  the  ventral  wall  of  the  foregut, 
first  one,  and  then  in  snccession  four  other  arterial  arches,  become  formed,  and  pass 
on  either  side,  one  into  each  visceral  arch.      Half  encircling   this   part   of   the 


Fig.  176  —  Diagrammatic  out- 
lines OP  THE  HEART  AKD  PRI- 
MITIVE VESSELS  OF  THE  EMBRYO 
CHICK  AS  SEEN  FROM  BELOW  AND 
ENLARGED.       (A.    T.  ) 

A,  soon  after  the  first  establish- 
ment of  the  circulation  ;  B,  C,  at 
a  somewhat  later  period  ;  1 ,  ] , 
the  veins  returning  from  the  vas- 
cular area  ;  2,  3,  4,  the  heart, 
now  in  the  form  of  a  notched 
tube  ;  5,  5  (upper),  the  two  primi- 
tive aortic  arches  ;  5,  5  (lower), 
the  primitive  double  aorta ;  a,  the 
single  or  united  aorta  ;  5',  5',  the 
continuation  of  the  double  aortse 
beyond  the  origin  of  the  large 
omphalo-mesenteric  arteries,  6,  6. 
The  division  above  4  is  repre- 
sented as  caiTied  rather  too  far 
down. 


alimentary  canal,  they  are  continued  above  it  into  two  descendinij  or  primitwe  aorke. 
These  two  vessels  run  down  the  trunk  on  either  side  of  the  notochord,  yielding,  as 
they  descend,  lateral  oflFsets  to  the  body  walls  and  to  the  yolk  sac.  Finally  they  give 
off",  at  the  lower  or  posterior  extremity,  two  large  vessels,  which  accompany  the 
allantois,  and  furnish  blood  to  the  foetal  part  of  the  placenta  (tmiMUcal  or  allantoic 
arteries). 

The  primitive  aortas  do  not  long  remain  double.  As  was  first  shown  by  means 
of  sections  by  Allen  Thomson,  they  unite  in  the  middle  line,  the  union  beginning 
in  the  dorsal  region  and  extending  forwards  and  backwards  ;  in  the  latter  direction 


DEVELOPMENT    oF    THE    PRINCIPAL    ARTEKIES. 


147 


even  Ix^yoml  tlie  (.rigiii  of  the  allantoic  arteries,  the  middle  sacral  artery  boinf;  in 
fact  the  extremity  of  the  aorta. 

Occasionally  the  imion  ivmaiiis  incoinpleto,  a  median  Foptum  bein^'  sometimes  found  as  a 
malformation  of  the  di'suendinjr  aorta. 

Tlie  com.non  iliacs  are  formed  by  persistence  of  the  roots  of  the  allantoic  arteries ;  when 
the  lower  limbs  are  formed  thoy  pive  otF  to  these  the  external  iliacs. 

Since  their  discovery  by  Kathke  in  182:,.  the  arterial  arches  have  been  regarded  with 
much  interest  as  corresponding  with  those  from  which  the  bloo<l-vessols  of  the  gills  in 
fishes  and  amphibia  are  derived.  Alon-  with  the  (subdivided)  aortic  bulb  they  give  rise  by 
various  tninsl urinations,  to  the  permanent  pulmonary  and  aortic  stems  and  the  principal  ves-sels 
wJiich  spring  from  them.  IMost  of  what  is  known  regarding  the  mode  of  their  transformation 
in  different  animals  is  due  to  the  researches  of  Itathke.  In  the  human  embryo  the  subject 
has  been  recently  investigated  by  His.  whose  account  will  be  here  mainly  followed. 


vo-co. 


VCV- 


-cUZ. 


Fig.  177. — Profile  view  of  a  human  embrto  of  about  fifteen  days,  with  the  alimextakt  caxal  in 

LONGITUDINAL  SECTION.       (His.) 

T'vo  arterial  arches  are  formed  at  this  stage. 


Fig.  17S.— Similar  view  of  a  sojiewuat  older  embrvo,  showing  five  arterial  arches. 
2,  3,  4,  5,  are  opposite  the  respective  secondai-y  cerebral  vesicles  ;  from  the  side  itf  the  fore-brain 


the  primary  optic  vesicle  is  seen  projecting  ;  ot,  otic  vesicle,  still  open  in  177  ;  js.i",  septum  between 
mouth  and  pharynx  (primitive  velum).  This  has  disappeared  in  178  ;  I,  commencing  liver  in  septum 
transversum  ;  \\  vitelline  stalk  ;  all,  allantois  enclosed  within  stalk  ;  j.v,  jugular  vein  ;  c.v,  cardinal 
vein;  s.r,  sinus  venosus  within  septum  transversum  ;  v..a,  left  umbilical  (allantoic)  artery;  l.v.v,  lef* 
umbilical  vein  ;  end.,  endothelial  tube  of  heart.  The  sharp  curve  of  the  trunk  of  the  embryo  towards 
tbe  yolk-sac  is  normal  at  this  period  cf  development. 


From  the  point  of  insertion  of  the  aortic  bulb  the  arterial  arches  have  a  radial 
disposition  as  they  pass  into  their  respective  visceral  arches  (fig.  181).  They  at  first 
effect,  as  above  stated,  a  complete  communication  between  the  aortic  bulb  and  the 
descending  aorta,  but  subsequently  iu  most  cases  the  communication  becomes 
obliterated,  and  the  completeness  of  the  vascular  arch  is  thus  obscured,  the  only 
arches  which  in  mammals  remain  pervious  through  their  whole  extent  up  to  the 

L  2 


148 


DEVELOPMENT    OF    THE    PRINCIPAL    ARTERIE,"^. 


time  of  birth  being  the  fourth  and  fifth  arches  of  the  left  side,  which  form  the  arch 
of  tlie  aorta  and  the  ductus  arteriosus  respectively.  This  obliteration  begins  early 
in  the  first  and  second  arches,  so  that  in  many  animals  by  the  time  the  posterior 
arches  are  formed  the  anterior  are  partially  obliterated  ;  but  in  man  this  is  not  the 
case,  all  five  pairs  of  arches  being  present  and  fully  pervious  for  a  certain  time  (figs. 
178,  179,  fi-om  His). 

As  development  proceeds,  the  point  of  insertion  of  the  aortic  Ijulb,  which  is  at 
first  opposite  the  first  arterial  arch,  becomes,  along  with  the  rest  of  the  heart, 


Fig.  179. — -Profile  view  of  a  human  embryo  of 

ABOUT  THREE  WEEKS,  SHOWING  THE  CEPHALIC 
A'ISCERAL  ARCHES  AND  CLEFTS  AND  THEIR  RELA- 
TIONS TO  THE  ARTERIAL  ARCHES.       (His.) 

mx,  maxillary  process  ;  tnn.  mandibular  arch  ; 
d.C,  duct  of  Guvier;  j.v,  jugular  vein;  c.v,  cardinal 
vein;  v.t,  vitelline  vein  ;  u.v,  umbilical  vein  ;  u.a, 
umbilical  artery  ;  all,  allantois  ;  ^j/,  placental  at- 
tachment of  allantoic  stalk  ;  olf,  olfactory  depres- 
sion ;  ot,  otic  vesicle. 

relatively  shifted  backwards,  and  on  each 
side  the  arterial  arches  presently  appear  to 
come  off  from  the  bulb  in  two  sets,  viz. : 
the  first  and  second  from  an  ascending 
trunk,  afterwards  the  external  carotid,  and 
the  third,  fourth,  and  fifth  from  a  descend- 
ing trunk. 

:As  the  point  of  insertion  is  still  further 
shifted,  the  third  arch  becomes  added  to 
the  ascending  trunk,  the  lower  part  of 
Avhich  now  forms  the  common  carotid. 
Finally,  by  a  continuance  of  the  same 
process,  the  fourth  arch  on  each  side  comes 
to  spring  from  the  ascending  trunk  :  that 
of  the  right  side  forming  the  innominate 
artery,  that  of  the  left  the  arch  of  the 
aorta  ;  and  only  the  fifth  arch,  from  which 
the  pulmonary  arteries  spring,  has  for  a 
time  a  descending  direction. 

From  the  dorsal  part  of  the  first  arch  a 
branch  passes  towards  the  brain — this 
becomes  the  upper  part  of  the  internal 
carotid.  "When  the  first  and  second  arches 
become  obliterated — a  change  which  next 
occurs — this  branch  remains  in  continuity 
with  the  third  arch  l)y  the  unobliterated  dorsal  portions  of  the  first  and  second 
arches  (upper  extremity  of  primitive  aort^)  :  these  portions,  together  with  the  third 
arch,  form  the  lower  part  of  the  internal  carotid,  the  posterior  communication 
between  the  third  and  fourth  arch  becoming  obliterated.  The  branches  of  the 
external  carotid  are  produced  from  the  remains  (ventral)  of  the  first  and  second 
arches  ;  the  maxillary  and  temporal  arteries  from  the  first,  the  lingual  and  ascending 
pharyngeal  arteries,  and  probably  also  the  occipital  and  auricular,  from  the  anterior 
part  of  the  second  arch. 

The  division  of  the  bulb  into  aortic  and  pulmonary  trunks  begins  just  at  the 
time  when  the  extremity  of  the  aortic  bulb  has  become  shifted  backward  so  as 


DEVELOPMENT    OF    THE    IMIIXCII'AL    AltTElllES. 


149 


to  ho  opposite  the  jwint  of  junction  hctwuen  the  I'ouilh  and  fifih  pairs  of  arches,  so 
tliat  now  all  the  arches  above  this  jioint  become  se])arated  oil"  in  connexion  with  the 
trunk  of  tlie  aorta  (ascending  aorta)  ;  the  one  below  it  remaining  ia  connexion  with 


Fi^.  180. — ^Profile  view  of  a  human  emrryo  of  about  3  or  1  weeks,  suowing  the  principal  arteries 

AND  veins.     (His.) 

1,  2,  3,  4,  5,  the  secondary  cerebral  vesicles  ;  hyp,  hypophysis  ;  ot,  otic  vesicle  ;  run,  mandibular 
arch  ;  bj.  lung  rudiment ;  st,  stomach  ;  Wd,  Wolffian  duct  opening  into  cloaca  ;  /,  //,  III,  IV,  V, 
the  arterial  arches',  springing  from  b.a,  bulbus  arteriosus  ;  p,  pulmonary  artery  ;  d.C,  duct  of  Cuvier  ; 
ch.,  notoi;urd. 

the  pulmonary  trunk.     After  the  separation,  the  aortic  bulb,  now  a  double  tube, 
becomes  still  further  shifted  back,  and  with  it  the  fourth  and  fifth  arches.     Since 


Fig.  181. — View  from  behind  of  the  anterior  part 

OF  THE  MOUTH  AND  PHARYNX  OF  A  HUMAN  EMBRYO 
OF  3i-  WEEKS.  SHOWING  THE  ARTERIAL  ARCHES 
RADIATING  FROM  THE  ATTACHMENT  OF  THE  AORTIC 
BULB.       (His.) 

ao,  point  of  attachment  of  aortic  bulb  in  the  anterior 
wall  of  the  pharynx  ;  ?hh,  In/,  ftr',  hr",  tirst  four  visceral 
arches  ;  /,  //,  ///,  IV,  the  corresponding  arterial 
arches  ;  V,  fifth  arterial  arch  giving  off  the  pulmonary 
artery, p. 

the  infei'ior  laryui^^eal  nerve  passes  under  the 
latter,  this  nerve  must  also  become  shifted 
downwards  along  with  these  arciies.  To 
allow  for  this  shifting  of  the  bulb,  the 
common  carotids  become  proportionally 
lengthened. 

From  the  descending  primitive  aorta3  on  either  side  a  series  of  inter-segmental 
arteries  pass  ;  the  uppermost  of  these  become  united  to  form  the  vertebral  arteries 
(which  subsequently  unite  superiorly  in  the  middle  line  to  produce  the  basilar),  the 
lower  form  intercostal  arteries.     A  branch  for  the  upper  extremity  comes  ofif  from 


150 


DEVELOPMENT    OF    THE    FOURTH    AND    FIFTH    ARCHES. 


the  commencement  of  the  vertebral  ;  it  subsequently  far  exceeds  the  parent  vessel 
in  size,  and  forms  the  subclavian,  this  name  being  extended  to  what  was  originally 
the  commencement  of  the  vertebral  from  the  descending  aorta. 

Destination  of  the  fourth  and  fifth  arches. — As  was  above  stated,  the 
aortic  trunk,  connected  below  with  the  left  ventricle,  is  connected  above  with  the 
four  superior  arches,  which  spring  from  its  two  ascending  rami  ;  of  these  rami  the 
right  is  much  the  smaller,  and  its  root  forms  the  innominate  ;  the  root  of  the  left, 
which  is  much  larger,  represents  the  part  of  the  arch  of  the  aorta  between  the 

Fig.  182  — Diagram  to  show 

THE     DESTINATION     OP     THE 
ARTERIAL     ARCHES     IN    MAN 

AND   MAMMALS.      (Modified 
from  Ratlike. ) 

The  truncus  arteriosus,  and 
the  five  arterial  arches  spring- 
ing from  it  are  represented  in 
outline  only,  the  permanent 
vessels  in  colours — those  be- 
longing to  the  aortic  system 
red,  to  the  pulmonary  system 
blue. 


siwclcovictft 


O/acerhdvriq/ 
cLordco 


—I'a^us  Tierve 
eoGtemaZ- 

,verteBrat 
otrcJi.  of 

sii^clcwimz 
ct/rteriosns 


sceruvirwf  cwrtco 


innominate  and  the  left 
common  carotid.  On 
the  right  side  the 
whole  of  the  fourth 
arch  remains,  and  forms  ~ 
the  common  subclavo- 
vertebral  trunk  ;  on  tlie 
left  side  the  whole  arch 
persists  and  forms  the 
remainder  of  the  arch  of 
the  aorta. 

The  fifth  arch  of  the 
right  side  only  persists 
as  far  as  the  origin  of 
the  branch  to  the  right 
lung,  the  remainder  of 
the  arch  disappears.  The  fifth  arch  of  the  left  side  jDcrsists  throughout  its  whole 
length  during  foetal  life,  and  joins  the  fourth  arch  as  the  ductus  arteriosus.  After 
birth,  the  part  of  the  arch  beyond  the  branch  to  the  corresponding  lung  becomes 
impervious  on  this  side  also,  and  is  converted  into  the  Ugamentum  arteriosum. 

The  upper  part  of  the  descending  primitive  aorta  disappears  entirely  on  the 
right  side  ;  that  of  the  left  side  forms  the  commencement  of  the  permanent 
descending  aorta. 

Rathke  described  both  pulmonary  arteries  as  being  given  off  from  the  fifth  left  arch  in 
mammals,  although  admitting  that  in  birds  and  reptiles  each  is  formed  from  the  corresponding 
arch.  In  birds  the  permanent  aortic  arch  is  the  fourth  arch  of  the  right  side,  and  not  of  the 
left  side  as  in  mammals,  and  in  reptiles  both  aortic  arches  remain  pervious. 

Many  of  the  abnormalities  which  are  observed  in  the  disposition  of  these  arteries,  may  be 
explained  by  regarding  them  as  a  persistence  of  embryonic  conditions. 

The  develoi^ment  of  the  arterial  arches  of  the  bird  has  been  recently  again  examined  by 
Mackay,  whose  account  differs  in  important  particulars  from  that  of  Rathke,  and  indeed  of 
nearly  all  previous  investigators.  He  describes  the  subclavian  artery  as  arising  from  the  third 
arch,  not  from  the  fourth  (this  was  also  given  as  its  origin  by  Sabatier),  springing  from  the 
ventral  part  of  the  arch,  and  running  outwards  superficial  to  the  pneumogastric  nerve  and  jugular 
vein.     The  third  arch  and  its  dorsal  upward  prolongation,  form  the  common  carotid,  not  the 


DEVELOPMENT   OF   THE   PKINUIFAL    VEINS.  151 

internal  carotid,  as  stated  by  llatlike,  and  the  ventral  prolongation  of  the  truncus  arterioauH 
forms,  not  the  external  earotid,  but  a  small  brancli  from  the  subolavian,  or  innominate 
artery,  to  the  front  of  tlie  trachea.  These  observations  have  not  been  as  yet  extended  to 
manunals. 

Zimmermann  lias  described,  both  in  the  niVjbit  and  in  the  human  (imbryo,  an  arterial  arch 
between  the  aortic  and  pulmonary  arches.  If  this  is  of  constant  occurrence  it  must  be 
reckoned  as  the  fifth  arch,  and  the  {>ulmonary  will  become  the  sixth. 

The  histogcnctic  changes  involved  in  the  development  of  the  blood-vessels  are  described  in 
the  chapter  on  Histology. 

The  tirst  vessels  appear,  as  has  been  already  stated,  in  the  mesoblast  of  the  vascular  area  ; 
the  lamina  of  mesoblast  in  which  they  are  formed  is  sometimes  distinguished  as  the  vascular 
lamina.  They  are  said  (by  His  and  others)  to  grow  inwards  from  tlie  va.scular  area,  but  the 
manner  in  which  the  principal  arteries  and  veins  of  the  body  are  first  developed  is  not  clear, 
beyond  the  fact  that  they  are  at  first  merely  endothelial  tubes.  The  muscular  tissue  of  the 
primitive  aortas  is  derived  from  the  lower  part  of  the  protovertebra  (I'].  Miiller). 

DEVELOPMENT     OF    THE    PRINCIPAL    VEINS. 

In  this  subject  ulso  the  description  given  by  His  of  the  condition  and  changes 
of  the  veins  in  the  human  embryo  will  be  followed,  although  it  differs  in  certain 
particulars  from  that  which  has  usually  been  received. 

In  the  early  embryo,  before  the  development  of  the  allantois,  two  vUellirui  or 
ompJmlo-nwseraic  veins,  right  and  left,  bring  back  the  blood  from  the  vascular  area 
upon  the  yolk  sac,  and  unite  to  form  a  common  trunk,  which  is  continued  as  the 
sinus  venosus  into  the  auricular  extremity  of  the  rudimentary  heart. 

At  the  commencement  of  the  placental  or  allantoic  circulation  (fourth  week  in 
man)  two  umhilical  veins  are  seen  coming  from  the  placenta  and  opening  into  the 
sinus  near  the  vitelUne  veins  (fig.  168).  Into  this  also  opens  on  either  side  a 
transverse  vein,  the  dud  of  Cuvier  or  superior  vena  cava,  which  is  formed  by  the  junc- 
tion of  the  'primitive  jugular  vein,  bringing  blood  from  the  head,  and  the  cardinal 
vein,  which  returns  the  blood  from  the  Wolffian  bodies,  tbe  vertebral  column  and 
the  body  walls  (figs.  179,  180).  The  trunk  or  sinus  into  which  all  these  veins  pour 
their  blood  is  now  transversely  disposed,  immediately  below  the  diaplu-agm,  and 
forms  the  saccus  reuniens  of  His,  which  has  been  already  alluded  to  (p.  139). 

The  vitelline  or  jomphalomeseraic  veins  enter  the  abdomen  along  the  vitelline 
duct  and  ascend  at  first  along  the  front  of  the  alimentary  canal,  but  higher  up  they 
are  seen  on  either  side  of  that  tube  (duodenum  and  stomach).  Here  transverse 
communications  form  between  the  two  veins,  two  in  front  of  and  one  behind  the 
duodenum,  so  that  this  is  encircled  by  two  vascular  rings  (figs.  180,  183).  Above 
these  venous  circles  the  direct  communication  with  the  sinus  becomes  lost,  the  inter- 
mediate venous  vessel  or  either  side  becoming  broken  up  within  the  substance  of  the 
liver  (which  has  by  this  time  developed  around  them)  into  a  vascular  network,  the 
middle  part  of  which  becomes  capillary. 

The  vessels  which  pass  from  the  upper  venous  ring  to  the  capillary  network  are 
known  as  venc&  advehentes,  they  become  the  branches  of  the  portal  vein ;  those 
which  pass  from  it  into  the  sinus  are  the  vcnce  revehcnlcs,  they  become  the  hepatic 
veins. 

The  lower  communication  between  the  vitelline  veins  takes  the  form  of  a  com- 
plete longitudinal  fusion  of  the  two  vessels,  at  least  for  some  distance.  This  fused 
part  receives  veins  from  the  intestine  and  stomach,  and  becomes  the  commencement 
of  the  portal  vein. 

The  umbilical  veins  are  for  a  long  time  double  within  the  abdomen,  although 
they  have  fused  within  the  umbilical  cord  into  a  single  trunk:  They  diverge  from 
this  and  pass  up  to  the  sinus  on  either  side  in  the  somatopleure,  just  where  this  is 
becoming  bent  round  into  the  amnion.  After  a  time,  however,  it  is  found  that  this 
direct  commuuication  with  the  sinus  is  partially  interrupted  by  the  development  of  a 


153 


DEVELOPMEISTT    OF    THE    PEINCIPAL    VEINS. 


vascular  network,  and  that  on  the  left  side  a  fresh  communication  has  become  esta- 
blished with  the  upper  venous  circle  of  the  vitelline  veins.  The  interruption  subse- 
quently becomes  complete  on  both  sides  (fig.  183),  and  on  the  right  side  the  greater 


V:u.ij 


VV^om. 


Fig.  183.- — Venous  trunks  of  a  human  embryo  of 

ABOUT  THREE-AND-A-HALF  WEEKS.       (His.) 

v.c.d,  v.c.s,  superior  vense  cavK,  right  and  left; 
r.j,  v.c,  iJrimitiA'e  jugular  and  cardinal  vein ; 
T.v.d,  v.u.s,  umbilical  veins,  right  and  left  ;  v.u", 
v.u",  upper  detached  portions  of  umbilical  veins  ; 
v.v.om,  omphalomeseraic  or  vitelline  veins  forming 
the  vena  portse.  The  permanent  veins  are  coloured 
blue. 


part"  of  the  vein  becomes  atrophied  (on 
both  sides  the  part  which  originally  opened 
into  the  sinus  reuniens  remains  evident 
for  a  time).  The  left  vein,  on  the  other 
hand,  increases  in  bulk  with  the  develop- 
ment of  the  placental  circulation.  For  a 
short  time  the  whole  of  its  blood,  as  well 
as  that  of  the  vitelline  vein,  passes  through 
the  capillaries  of  the  liver.     But  a  branch 


is  soon  seen  passing  from  the  upper  venous  circle  direct  into  the  right  hepatic 
vein,  near  its  entrance  into  the  sinus.  This  forms  the  ductus  venosus  or  vena 
asce7idens,  and  it  now  carries  most  of  the  blood  of  the  umbilical  vein  direct  to  the 
heart.     Subsequently  the  direct  communication  of  the  left  hepatic  vein  with  the 


Fig.  184. — Under  surface  op  the  fcetal  liver, 

WITH    ITS    great    BLOOD-TESSELS,    AT    THE    FULL 
PERIOD. 

a,  the  umbilical  vein,  lying  in  the  umbilical 
fissure,  and  turning  to  the  right  side,  at  the  trans- 
verse fissure  (o),  to  join  the  vena  i3ort8o(  jj)  ;  d,  the 
ductus  venosus,  continuing  straight  on  to  join  the 
vena  cava  inferior  (c) ;  some  branches  of  the  umbili- 
cal vein  pass  from  a  into  the  substance  of  the  liver  ; 
g,  the  gall-bladder,  cut. 


sinus  becomes  obliterated,  and  a  new  com- 
munication becomes  established  with  the 
ductus  venosus  ;  and,  finally,  when,  with 
the  growth  of  the  lower  hmbs  and  of  the 
other  abdominal  and  pelvic  organs,'  the  inferior  vena  cava  becomes  developed,  this 
also  joins  the  upper  end  of  the  ductus  venosus. 

The  lower  part  of  the  portal  vein  is  formed,  as  we  have  seen,  by  the  united 
vitelhne  veins.  The  upper  part  is  formed  as  a  single  trunk  out  of  the  double  venous 
annulus  by  atrophy  of  the  right  half  of  the  lower  ring  and  the  left  half  of  the  upper 
(fig.  183).  The  spiral  turn  around  the  duodenum  is  thus  produced,  and  thus  it  is 
also  that  the  portal  vein  at  first  appears  more  directly  connected  with  the  right  vense 
advehentes  than  with  the  left. 

Most  of  these  embryonic  veins  are  at  first  of  relatively  large  size  and  have  an 
irregular  sinus-hke  character,  which  disappears  at  a  later  stage  of  development. 

At  the  time  of  commencement  of  the  j)lacental  circulation,  two  short  transverse 
venous  trunks,  the  ducts  of  Guvier,  open,  as  has  been  above  stated,  one  on  each  side, 
into  the  sinus  venosus  of  the  heart.  Each  is  formed  by  the  union  of  a  superior  and 
an  inferior  vein,  named  respectively  ih.Q  primitive  jugular  and  the  cardincd. 


l)KVKlA>l'.Mi:\T    OF    THK    l'i:i.\(  1 1'.\  I,    VKIXS. 


153 


'V\\v  pritiiitirv  jiKjiilid-  vein  ivtTive'S  tlu;  hlood  from  the  (•niiiial  caviLy  by  cliiuiiiels 
ill  fiont  (tf  tlio  ear,  wliicli  are  sultseqiiontly  obliterated  :  in  the  ;;Tealer  part  of  its 
extent  it  l)ec(inies  the  external  jnuular  vein  ;  and  near  its  lower  end  it  receives  small 
branches,  which  <;row  to  be  the  internal  jii;ruhir  and  sul)clavian  veins  (fig.  IHij). 
The  cardinal  reins  are  tiie  primitive  vessels  which  retnrn  the  blood  from  the 
Wolffian  bodies,  the  vertebral  column,  and  the  parietes  of  the  trunk.  Their  lower 
extremities,  which  become  developed  later  into  the  internal  iliac  veins,  receive,  cs 


ImU.    lSr>. — SCIIKMK  OK  TIIK   IiKVKLOl'MKNT    uK    Tlir.    f'Hrr.K  VKIXS 
OF    TIIK    HUDV.        ((t.    D.   T.  ) 

TIic  primitive  venous  trunks  are  imlicateil  liy  black  outlines, 
and  their  names  are  enclosed  witliin  parentheses-.  The  delinitive 
veins  are  represented  blue. 

the  lower  limb  becomes  developed,  the  sciatic  veins, 
and  later  the  external  iliac  ;  hi^-her  up,  with  the 
development  of  the  kidneys,  the  renal  veins,  with 
the  spermatic  and  suprarenal,  also  open  into  them. 
The  bilaterally  symmetrical  arrano-emeut  which  at 
first  prevails  becomes  lost,  owinii;-  to  the  obliteration 
of  a  large  part  of  the  left  cartlinal  vein,  and  the 
formation  of  a  transverse  communication  {trans- 
verse iliac  vein)  between  its  lower  extremity  and 
the  right  cardinal,  a  little  above  the  place  where 
the  right  exterual  iliac  joins  it.  The  greater  part 
of  the  left  common  iliac  becomes  formed  by  this 
communicating  vessel,  only  a  small  part  being 
formed  by  cardinal  vein,  while  the  right  is  wholly 
formed  by  cardinal  vein. 

Above  the  transverse  iliac  vein  the  fate  of  the 
cardinal  veins  on  the  two  sides  is  veiy  different. 
That  of  the  left  side  becomes  obliterated  for  some 
distance,  whereas  that  of  the  right  side  undergoes 
an  increased  development,  receiving  as  it  now  does 
the  blood  from  the  whole  of  the  lower  limbs  and 
pelvis  :  it  forms  the  greater  part  of  the  inferior 
vena  cava.  But  the  hepatic  part  of  the  vena  cava 
has  become  developed  independently  of  the  cardinal 
veins  as  a  median  vessel  which  opens  above  into  the 
tnmk  formed  by  the  junction  of  the  ductus  venosus 
with  the  vitelline  veins.  A  commmiicating  branch 
is  soon  formed  on  either  side  between  the  lower 
end  of  this  median  vessel  and  the  cardinal  veins 
at  the  place  where  they  receive  the  renal  veins, 
and  on  the  obliteration  of  the  lower  part  of  the  left  cardinal  the  left  renal  vein 
opens  by  this  communicating  branch  directly  into  the  vena  cava.  On  the  right  side 
the  communication  becomes  the  continuation  of  the  main  trunk,  for  the  right 
cardinal  also  becomes  obliterated  for  a  short  distance  above  the  renal  vein.  (See 
also  Vol.  II.,  Part  2,  "  Morphology  of  the  Venous  System,")  ^ 

As  development  proceeds,  the  direction  of  the  ducts  of  Cuvicr  is  altered  by  the 
descent  of  the  heart  from  the  cervical  into  the  thoracic  region,  and  becomes  the  same 

^  Advantage  lias  been  taken  of  the  necessity  of  reprinting  this  volume  to  alter  the  account  of  the 
development  of  the  veins  of  the  trunk,  the  former  description  having  been  based  upon  the  earlier  re- 
searches of  Rathke,  which  have  been  show-n  by  Hochstetter  to  require  moditication  (November,  1895). 


154 


DEVELOPMENT    OF    THE    PEINCIPAL    VEINS. 


as  that,  of  tlie  primitive  jugular  yeins.  A  communicating  branch  (which  may  be 
termed  the  transverse  jugular)  makes  its  appearance,  directed  transversely  from  the 
junction  of  the  left  subclavian  and  jugular  veins,  across  the  middle  line  to  the  right 
jugular;  and  further  down  in  the  thoracic  region  between  the  cardinal  veins,  a 
communicating  branch  {transverse  cardinal)  passes  obliquely  across  the  middle  line 
from  right  to  left.  The  communicating  branch  betw^een  the  primitive  jugular  veins 
is  converted  into  the  left  brachiocephalic  or  innominate  vein.  The  portion  of  vessel 
between  the  right  subclavian  vein  and  the  termination  of  the  transverse  jugular 
becomes  the  riglit  brachiocephalic  vein.  The  portion  of  the  primitive  jugular  vein 
below  the  transverse  jugular,  together  with  the  right  duct  of  Cuvier,  forms  the  vena 
cava  superior,  while  the  cardinal  vein  opening  into  it  is  the  extremity  of  the  great 


Fig.  186. — A  and  B. — Diagrammatic   outlines  of  the  vestige  of  the  left  superior  cava  and  of 
A  CASE  of  its  persistence.     (Sketched  after  Marshall.)     I. 

The  views  are  supposed  to  be  from  before,  the  parts  of  the  heart  beiog  removed  or  seen  through. 

1,  1',  internal  jugular  veins  ;  2,  2',  subclavian  veins  ;  3,  right  innominate  ;  3',  right  or  regular 
superior  cava  ;  4,  left  innominate,  normal  in  A,  rudimentary  in  B  ;  5,  in  A,  the  opening  of  the  superior 
intercostal  vein  into  the  innominate  ;  5',  vestige  of  the  left  superior  cava  or  duct  of  Cuvier  ;  5,  5',  in  B, 
the  left  vena  cava  supeiior  abnormally  persistent  ;  6,  coronary  sinus  ;  6',  coronary  veins  ;  7,  superior 
intercostal  trunk  of  the  left  side  (left  cardinal  vein)  ;  8,  the  jjrincipal  azygos  (right  cardinal  vein) ; 
7',  8',  some  of  the  upper  intercostal  veins  ;  9,  the  opening  of  the  inferior  vena  cava,  vith  the  Eustachian 
valve. 

vena  azygos.  On  the  left  side,  the  portion  of  the  primitive  jugular  vein  placed 
below  the  transverse  jugular  may  still  be  represented  by  the  left  superior  intercostal 
vein,  but  the  duct  of  Cuvier  becomes  obliterated  (with  the  exception  of  the  part 
which  forms  the  coronary  sinus)  as  does  also  usually  the  uppermost  part  of  the 
cardinal  vein.  The  remaining  part  of  the  lefc  cardinal,  however,  which  receives  most 
of  the  left  intercostal  veins,  becomes,  with  the  transverse  cardinal,  converted  into  the 
left  azygos  veins.  The  variability  in  the  adult  arrangement  of  the  veins  depends 
on  the  different  extent  to  which  the  originally  continuous  vessels  are  developed  or 
atrophied  at  one  point  or  another.  The  left  duct  of  Cuvier  is  obliterated.  But  even 
in  the  adult,  traces  of  this  vessel  can  ahvays  be  recognised  in  the  form  of  a  fibrous 
band,  or  sometimes  a  narrow  vein,  which  descends  obliquely  over  the  left  auricle  ; 
and  in  front  of  the  root  of  the  left  lung  there  remains  an  indication  of  its  former 
presence  in  the  form  of  a  small  fold  of  the  serous  membrane  of  the  pericardium,  the 


rECULlAi:iTIi:s    Ul'    TUK    F(KTAL    organs    of    CIIICLILATIoX.         lo.j 

vest  if/ id  I  fold  of  ^[arsliall,  to  wliom  is  due  tlic  first  full  elucidation  of  the  nature  and 
relations  of  the  left  j)riniitive  vena  cava  superior. 

The  left  duct  of  Cuvier  lias  been  ol)ser\ed  jiersistent  as  a  small  vessel  in  the 
adult.     Jiess  frequently  a  ri^ht  and  a  left  innominate  vein  open  separately  into  the 

Fig.    187.  — Vii:w    ok   thi:    kiktai.    iikakt    ani>   <;iikat 
vks8ki.s,  krom  tuk  i.kkt  s1i>k,  to  slkiw  thk  vksthik 

OK  THK    I.KKT    Sll'KUIoK  VKNA  CAVA  IN  SITU.       (TluH 

ligure  is  iiliiniR'd  after  one  of  MjhsIimU's. ) 

a,  rijjht  nmiclo  ;  h,  left  auricle  and  imliiioiijiry  veins  ; 
c,  the  coiius  arteriosus  of  tiic  ri^'ht  ventricle  ;  </,  the  left 
ventricle  ;  c,  descending  aorta  ;  I  ,  vestigial  fold  of  the 
pericardium  ;  /,  arcli  of  the  aorta,  with  a  part  of  the 
pericardium  rcmuininj<  aliove;  (f,  main  pulmonary  artery 
.and  ductus  arteriosus  ;  ;/',  left  pulmonary  artery  ;  1,  T, 
right  and  left  internal  jugular  veins  ;  li,  2',  suhchivian 
veins  ;  3,  .'!',  riglit  innondnate  and  superior  vena  cava  ; 
■1,  left  iniuiminate  ;  5,  ;")',  remains  of  the  left  superior 
cava  anil  duct  of  Cuvier,  passing  at  +  in  the  vestigial 
fold  of  the  peric-.irdium,  joining  the  coronary  sinus,  0, 
below,  and  receiving  above  the  superior  intercostal  vein, 
7  ;  7',  7',  the  upper  and  lower  intercostal  vein. 

n\2;ht  auricle,  an  arraugcmcnt  which  is  also  met  with  in  bii'ds  and  in  certain 
maiuinals,  and  which  results  from  the  vessels  of  the  left  side  beiuf;'  developed 
similarly  to  those  of  the  ri,o-ht,  while  the  cross  branch  remains  small  or  absent. 

A  ease  is  recorded  by  GrubL'r  in  which  the  left  vena  azygos  opened  into  the 
coronary  sinus,  and  was  met  by  a  small  vein  descending  from  the  union  of  the 
subclavian  and  jugular.  Here,  then,  the  jugular  veins  had  been  developed  in  the 
usual  manner,  while  the  left  vena  azygos  continued  to  pour  its  blood  into  the  duct 
of  Cuvier. 


PECULIARITIES    OF    THE    FCETAL    ORGANS    OF    CIRCUIiATION. 

It  may  be  useful  here  to  recapitulate  shortly  the  peculiarities  of  structure  exist- 
ing in  the  advanced  stage  of  the  formation  of  the  foetal  organs  of  circulation,  with 
reference  to  their  influence  in  determining  the  course  of  the  blood  during  intra- 
uterine life,  and  the  changes  w^hich  occur  in  them  upon  the  establishment  of  pulmo- 
nary respiration  at  birth. 

The  foramen  ovale  retains  the  form  of  a  free  oval  opening  in  the  septum  atri- 
oruni  up  to  tlie  fourth  month,  but  in  the  course  of  that  month  and  the  next  the 
growth  of  the  valvular  plate  which  fills  np  the  floor  of  the  fossa  ovalis,  becomes  com- 
plete, so  that  in  the  last  three  and  a  half  mouths  the  blood  can  only  pass  from  the 
right  into  the  left  auricle,  not  in  a  contrary  direction. 

The  Eustachian  valve  constitutes  a  crescentic  fold  of  the  lining  structure  of 
the  heart,  which  is  so  situated  as  to  direct  the  blood  entering  the  auricle  by  the  in- 
ferior cava  towards  the  opening  of  the  foramen  ovale. 

The  ductus  arteriosus  establishes  a  communication  lietwecn  the  main  pul- 
monary artery  and  the  aorta,  by  which  the  blood  from  the  right  ventricle  is  carried 
mainly  into  the  dorsal  aorta. 

The  two  large  hypogastric  oi-  umbilical  arteries,  prolonged  from  the  iliac 
arteries,  passing  out  (jf  the  body  of  the  fcetus,  proceed  along  the  umbilical  cord,  to 
be  distributed  in  the  ttetal  portion  of  the  placenta.  From  the  placenta  the  blood  is 
returned  by  the  umbilical  vein,  which,  after  entering  the  abdomen,  communicates 
by  one  branch  with  the  portal  vein,  and  is  continued  by  another,  named  ductus 
vmosus,  into  one  of  the  hepatic  veins,  through  which  it  joins  the  main  stem  of  the 
vena  cava  inferior. 

Course  of  the  blood  in  the  fcetus. — The  riuht  auricle  of  the  foetal  heart 


156 


COURSE    OF    THE    BLOOD    IN    THE    FCETUS. 


V? 


receives  blood  from  the  two  vena  cavse  and  the  coronary  sinus.  The  blood  brought 
by  the  superior  cava  is  simply  the  venous  blood  returned  from  the  head  and  upper 
half  of  the  body  ;  whilst  the  inferior  cava,  which  is  considerably  larger  than  the 
superior,  conveys  not  only  the  blood  from  the  lower  half  of  the  body,  but  also  that 
which  is  returned  from  the  placenta  and  the  liver.  This  latter  stream  of  blood 
reaches  the  vena  cava  inferior,  partly  by  a  direct  passage — the  ductus  venosus — and 
partly  by  the  hepatic  veins,  which  bring  to  the  vena  cava  inferior  aU  the  blood 

Fig.  188. — Diagrammatic  outline  op  the 

ORGANS    OF    CIRCULATION    IN    THE    PffiTUS 

OF  SIX  MONTHS.     (Allen  Thomson.  )i 

RA,  right  auricle  of  the  heai't ;  EV,  right 
ventricle  ;  LA,  left  auricle  ;  E^^,  Eustachian 
valve  ;  LV,  left  ventricle  ;  L,  liver  ;  K,  left 
kidney  ;  I,  portion  of  small  intestine  ;  a, 
arch  of  the  aorta  ;  a',  its  dorsal  part ;  a", 
lower  end  ;  vcs,  superior  vena  cava  ;  vci,  in- 
ferior vena  v/here  it  joins  the  right  auricle  ; 
i'ci',  its  lower  end  ;  s,  subclavian  vessels  ; 
_/,  right  jugular  vein  ;  c,  common  carotid 
arteries  ;  four  curved  dotted  arrow  lines  are 
carried  through  the  aortic  and  pulmonary 
opening,  and  the  auriculo-veutricular  orifices ; 
da,  opposite  to  the  one  passing  through  the 
pulmonary  artery,  marks  the  place  of  the 
ductus  arteriosus  ;  a  similar  arrow  line  is 
shown  passing  from  the  vena  cava  inferior 
through  the  fossa  ovalis  of  the  right  auricle, 
and  the  foramen  ovale  into  the  left  auricle  ; 
liv,  the  hepatic  veins  ;  wp,  vena  portae  ;  x  to 
vol,  the  ductus  venosus  ;  ilv,  the  umbilical 
vein  ;  im,  umbilical  arteries  ;  mc,  umbilical 
cord  cut  short  ;  I  i',  iliac  vessels. 

circulating  through  the  liver, 
whether  derived  from  the  supply 
of  placental  blood  entering  that 
organ  by  the  umbilical  vein,  or 
proceeding  from  the  vena  portte  or 
hepatic  artery. 

The  blood  of  the  superior  vena 
cava  is  believed  to  pass  through  the 
right  auricle  into  the  right  ventricle, 
whence  it  is  propelled  into  the  trunk 
of  the  pulmonary  artery,  A  small 
part  is  distributed  through  the 
branches  of  that  vessel  to  the  lungs, 
and  returns  by  the  pulmonary  veins 
to  the  left  auricle  ;  but,  as  these 
vessels  remain  small  up  to  the  time 
of  birth,  by  far  the  larger  part  passes 
through  the  ductus  arteriosus  into  tne  descending  aorta,  and  is  thence  distributed  in 
part  to  the  lower  half  of  the  body  and  the  viscera,  and  in  part  along  the  umbilical 
arteries  to  the  placenta.  From  these  several  organs  it  is  returned  by  the  vena  cava 
inferior,  the  vena  port^,  and  the  umbiUcal  vein ;  and,  as  already  noticed,  reaches 
the  right  auricle  through  the  trunk  of  the  inferior  cava. 

Of  the  blood  entering  the  heart  by  the  inferior  vena  cava,  it  is  supposed  that 
only  a  smaU  part  is  mingled  with  that  of  the  superior  cava,  so  as  to  pass  into  the 

1  The  red  and  blue  coloration   of  this  diagram  represents  the  fact  that  the  vessels  coloured  are 
arteries  and  veins  respectively,  and  not  the  quality  of  blood  which  they  convey. 


CUANGKS    IN    THE    CIRCULATION    AT    BlltTlI.  l5? 

riglit  ventricle;  by  far  Llie  lari;cr  poilioii  is  tliuui;lit  to  be  direeted  by  the  Eustachian 
valve  tlirouf!:h  the  foramen  ovale  into  the  left  auricle,  and  thence,  together  with  the 
small  quantity  of  blood  returned  from  tlie  luniks  by  the  pulmonary  veins,  to  pass 
into  the  left  ventricle,  whence  it  is  sent  into  the  arch  of  tlu!  aorta,  to  be  distributed 
almost  entirely  to  the  head  and  upper  liinl)s. 

Sabatier  wasthe  lirst  tocall  attention  particularly  to  the  action  of  the  Eustachian 
valve  in  separatiuj;:  the  currents  of  blood  entering  the  right  auricle  by  the  superior 
and  inferior  vena;  c;\\x.  Tiiis  scjiaration,  as  well  as  that  occurring  between  the 
currents  passing  through  the  aortic  arch  and  the  ductus  arteriosus  into  the  descend- 
ing aorta,  was  illustrated  experimentally  l)y  John  Reid.  A  striking  confirmation  of 
the  extent  to  which  the  last  mentioned  division  of  the  two  currents  of  the  foetal 
blood  may  take  place,  without  disturbance  of  the  circulation  up  to  the  time  of  birth, 
is  afforded  by  the  examples  of  malformation  in  which  a  complete  obliteration  has 
existed  in  the  aortic  trunk  immediately  before  the  place  of  the  union  of  the  ductus 
arteriosus  with  the  posterior  part  of  the  aortic  arch. 

In  earlier  stages  of  development  than  those  above  described,  it  is  certain  that 
there  is  little  or  no  separation  of  the  two  kinds  of  blood,  for  both  the  umbilical 
veins  from  the  placenta  and  the  veins  from  the  yolk  sac  and  body  generally,  pour 
their  blood  together  into  the  sinus  vonosus,  and  the  mixed  blood  is  then  forced 
through  a  single  somewhat  narrowed  orifice  {poria  vei^tilinli  of  Ilk)  into  the  auricle. 

CHANGES    IN    THE    CIRCULATION    AT    BIRTH. 

The  changes  which  occur  in  the  organs  of  circulation  and  respiration  at  birth, 
and  which  lead  to  the  establishment  of  their  permanent  condition,  are  more  immedi- 
ately determined  by  the  inflation  of  the  lungs  with  air  in  the  first  respiration,  the 
accompanying  rapid  dilatation  of  the  pulmonary  blood-vessels  with  a  greater  quantity 
of  blood,  and  the  interruption  to  the  passage  of  blood  through  the  placental  circula- 
tion. These  changes  are  speedily  followed  by  shrinking  and  obliteration  of  the 
ductus  arteriosus,  and  of  the  hypogastric  arteries  fi'om  the  iliac  trunk  to  the  place  of 
their  is^ue  from  the  body  by  the  umbilical  cord  ;  by  the  cessation  of  the  passage  of 
blood  through  the  foramen  ovale,  and  somewhat  later  by  the  closure  of  that  foramen, 
and  by  the  ol)literation  of  the  uml)ilical  vein  as  far  as  its  entrance  into  the  hver,  and 
of  the  ductus  venosns  behiiid  that  organ. 

The  process  of  obliteration  of  the  arteries  appears  to  depend  at  first  mainly  on 
the  contraction  of  their  coats,  but  this  is  very  soon  followed  by  a  considerable 
thickening  of  their  substance,  reducing  rapidly  their  internal  passage  to  a  narrow 
tube,  and  leading  in  a  short  time  to  final  closure,  even  although  the  vessel  may  not 
present  externally  any  considerable  diminution  of  its  diameter.  It  commences  at 
birth,  and  is  perceptible  after  a  few  respirations  have  occurred.  It  makes  rapid 
progress  in  the  first  and  second,  days,  and.  by  the  third  or  fourth  day  the  passage 
through  the  umbilical  arteries  is  usually  completely  interrupted.  The  ductus 
arteriosus  is  rarely  found  open  after  the  eighth  or  tenth  day,  and  by  three  weeks  it 
has  in  almost  all  instances  become  completely  impervious. 

The  process  of  closure  in  the  veins  is  slower  ;  but  they  remain  empty  of  blood 
and  collapsed,  and  by  the  sixth  or  seventh  day  are  generally  closed. 

Although  blood  ceases  at  once  to  pass  through  the  foramen  ovale  from  the 
moment  of  birth,  or  as  soon  as  the  left  auricle  becomes  filled  with  the  blood  return- 
ing from  the  lungs,  and  the  pressure  within  the  two  auricles  tends  to  be  more 
equalised  during  their  diastole,  yet  the  actual  closure  of  the  foramen  is  more  tardy 
than  any  of  the  other  changes  now  referred  to.  It  is  gradually  effected  by  the  union 
of  the  forepart  of  the  valve  of  the  fossa  ovalis  with  the  margin  of  the  limbus  of 
Yieussens  on  the  left  side  j  but  the  crescentic  margin  is  generally  perceptible  in  the 


158  THE   LYMPHATIC    SYSTEM. 

left  auricle  as  a  free  border  beyond  tbe  place  of  union,  and  not  unfrequently  the 
union  remains  incomplete,  so  that  a  probe  may  be  passed  through  the  reduced 
aperture.  In  many  cases  a  wider  aperture  remains  for  more  or  less  of  the  first  year 
of  infancy,  and  in  certain  instances  there  is  such  a  failure  of  the  union  of  the  valve 
as  to  allow  of  the  continued  passage  of  venous  blood,  especially  when  the  circulation 
is  disturbed  by  over-exertion,  from  the  right  to  the  left  auricle,  as  occurs  in  the 
malformation  attending  the  morbus  coeruleus. 


THE     LYMPHATIC     SYSTEM. 

Tlie  clevelojjment  of  the  lymphatic  system  has  been  studied  in  the  chick  by  Budg-e.  Here 
there  exist  a  network  of  lymphatics  agreeing  in  their  general  distribution  with  the  blood- 
vessels in  the  vascular  area.  This  is  the  first  lymphatic  circulation  ;  a  second  one  is  formed 
later,  corresponding  with  the  allantoic  circulation.  The  lymphatics  of  both  systems  communi- 
cate with  the  coelom,  but  only  those  of  the  second  circulation  communicate  with  veins.  The 
lymphatics  lie  in  the  vascular  area  above  and  close  to  the  blood-vessels,  but  are  separated 
near  the  embryo  by  a  layer  of  meso  blast  continuous  with  the  splanchnopleure. 

The  method  of  development  of  lymphatic  vessels  and  lymphatic  glands  is  dealt  with  in 
the  chapter  on  Histology. 


RECENT    LITERATURE, 

Blaschek,  A.,  UntersucTiung  ilher  Eerz,  Pericard,  Endocard  und  Pericardialhohle,  Mittheil. 
aus  dem  embryol.  Institut  der  Universitat  Wien,  1885. 

Born,  Gr.,  Entu-icUung  des  Sdugethier-herzens,  Archiv  f.  mikr.  Anat.,  Bd.  33,  1889. 

Budg-e,  A. ,  Untersuchungen  ilher  die  Entwickelung  des  Lymjplisy stems  beirn  Huhneremhryo,  Arch, 
f.  Anat.  u.  Physiol.,  Anat.  Abtheil.,  1887. 

Fiirstig-,  J.,  Untersuchungen  iiher  die  Entioiclcelung  der  primitiven  Aorten  mit  hesondcrer 
Berilchsichtigung  der  Beziehungen  derselhen  zu  den  Anlagen  des  Herzens,  Schriften  herausgegeben  von 
der  Naturf.  Gesellsch.  bei  der  Univers.  Dorpat,  i.,  1884. 

G-reenfield,  W.  S.,  Case  of  malformation  of  the  heart,  djc,  .Journal  of  Anat.  and  Physiol.,  1890. 

Hochstetter,  F.,  Beitrage  zur  Enticiclcclungsgeschichte  des  Venensy sterns  der  Amnioten,  Morpholog. 
Jahrbuch,  Bd.  xiii.,  1888. 

Mackay,  J.  Yale,  The  development  of  the  branchial  arterial  arches  in  birds,  with  special  refer- 
ence to  the  origin  of  the  subclavians  and  carotids,  Philosophical  Transactions,  B,  1888. 

Marius,  J.,  Quelques  notes  sur  le  diveloppement  du  cceur  chez  le  poulet,  Arch,  de  biol.,  1889. 

Mayer,  P.,  Ucber  die  EntwicJcelung  des  Herzens  und  der  grossen  Gefdssstdmme  bei  den  SeUuhiern, 
Mitth.  d.  zool.  Station  zu  Neapel,  Bd.  vii.,  1887. 

Miiller,  Erik,  Studien  iiber  den  Ursprung  der  Gefdssmushulatur,  Archiv.  f.  Anat.  u.  Physiol. 
Anat.  Abth.,  1888. 

Rabl,  C,  Ueher  die  Bildung  des  Herzens  der  Amphibien,  Morphol.  Jahrbnch,  1886  ;  Veber  die 
Bildung  der  Herzanlage,  Wiener  Medicin.  Presse,  1886. 

Rose,  C,  Beitrage  zur  EntvAclcclungsgeschichte  des  Herzens,  Dissert.  Heidelberg,  1888,  and  Morphol 
Jahrb.  xv. 

Kiickert,  J.,  Veber  den  Ursprung  des  Herzendothels,  Anat.  Anzeiger.  1887  ;  Ueber  die  Entstehung 
der  endothelialen  Anlagen  des  Herzens  und  der  ersten  Gefdssstdmme  bei  Selachier-Ernbryonen,  Biolog. 
Centralb.,  Bd.  viii.,  1888. 

Sckimkewitscli,  "W.,  Ueber  die  Identitdt  der  Herzbildwng  bei  den  Wirbel-  und  wirhellosen 
Thieren,  Zoolog.  Anz.,  1885. 

Sch-wink,  F.,  Ueber  die  Entwickelung  der  Herzendothels  der  Amphibien,  Anat.  Anzeiger,  1890. 
Tiirstig-,  J.,  Mittheilungen  iiber  die  Entwickelung  der  primitiven  Aorten  ruich  Untersuchungen  an 
Huhnerembryonen,  Dissert.  Dorpat.,  1886. 

Van  Bemmelen,  J.  F.,  Die  Visceraltaschen  und  Aortenhogen  bei  Reptilien  und  Vogdn,  Zool. 
Anzeiger,  1886. 

Zimmermann,  W.,  Ueber  ein  zwischen  Aorten-  «.  Pubnonalbogen  gdegenen  Kiemeaarterienbogen 
beim  Kaninchen,  Anat.  Anzeiger,  1889. 


DEVELOPMENT   OF   THE   MUSCLES.  139 


DEVELOPMENT     OF     THE     SEROUS     CAVITIES     AND     OP     THE 
MUSCLES     AND     SKELETON. 

The  seroxis  cavities — peritoneum,  pleiine,  pericardium — arc  derived  from  the 
original  split  or  cleavage  of  the  mesoblast,  which  constitutes  the  coelom  or  general 
body  cavity  (pleurn-peritoncal  cavity  of  older  authors).  This  cleft  is  formed  in  the 
head  as  well  as  in  tlie  trunk,  and  wlien  the  heait  is  formed  jus  a  double  tube,  each 
half  is  enclosed  within  a  portion  of  that  cavity,  which  hiter  on,  when  the  body  walls 
bend  round  and  meet  to  enclose  tlie  fore-gut,  comes,  like  the  heart  itself,  to  occupy  a 
position  on  the  ventral  aspect  of  the  alimentary  tube.  The  part  of  the  ccelom  which 
thus  contains  the  heart  is  not  for  some  time  entirely  distinct,  but  communicates 
dorsally  by  two  comparatively  narrow  channels  with  the  anterior  part  of  the  general 
body  cavity,  here  separated  into  lateral  halves,  which  ultimately  become  the  pleura3,' 
by  the  alimentary  can:il,  Subse(|uently  these  communications  become  obliterated, 
and  the  heart-coelom  separated  as  a  distinct  cavity  (pericardial  cavity).  Below, 
where  the  great  veins  enter  the  heart,  they  pass  into  a  mass  of  mesoblastic  tissue, 
which  is  connected  with  the  anterior  body  wall  (where  it  receives  the  umbilical 
and  vitelline  veins),  and  also  with  the  lateral  wall  (where  it  receives  the  ducts  of 
Cuvier),  and  which,  as  the  heart  bends,  so  that  the  venoiLS  end  passes  behind  the 
ventricle  and  bulb,  is  carried  along  with  the  veins  up  behind  that  organ,  and  thus 
forms  an  obliquely  placed  thick  septum,  at  first  incomplete,  but  subsequently  becoming 
entirely  closed,  which  separates  the  heart  within  the  pericardial  part  of  the  body 
cavity  in  front  and  above  from  the  stomach  and  alimentary  canal  within  the  peri- 
toneal part  of  that  cavity  behind  and  below.  The  thick  septum,  besides  containing 
the  saccus  reuniens  and  the  portions  of  the  great  veins  (vitelline,  umbilical,  ducts  of 
Cuvier)  which  open  into  that  cavity,  also  contains  the  rudiments  of  a  part  of  the 
diaphragm  and  the  mesoblastic  part  of  the  liver,  into  which  the  hypoblastic  part 
'grows  from  the  adjacent  duodenum  ;  it  has  been  termed  by  His  the  transverse  septum 
(see  figs.  177,  178).  As  development  proceeds,  the  septum  becomes  gradually  difie- 
rentiated  into  its  several  parts.  The  great  veins  become  still  further  shifted  behind 
the  heart,  and  the  saccus  reuniens  becomes  incorporated  with  that  organ.  The 
liver,  which  is  at  first  contained  entirely  within  the  septum,  becomes  split  ott*  from 
its  upper  layer,  which  now  forms  the  thin  portion  of  the  diaphragm,  while  the  cavity 
of  the  peritoneum  extends  from  cither  side,  and  separates  them  from  one  another, 
except  along  the  attachment  of  the  broad  ligaments. 

The  diaphragm  is  completed  by  a  growth  of  mesoblast  which  occurs  on 
each  side,  and  cuts  off  the  antero-dorsal  portions  of  the  body-cavity  into  which 
the  lungs  are  invaginated  (recessus  pulmonales)  from  the  posterior  or  peritoneal 
part. 

The  serous  membranes  are  formed  by  differentiation  of  the  lining  mesoblast  of 
the  coelom. 

The  formation  of  the  omenta,  and  the  changes  which  the  mesenteric  folds  of 
peritoneum  undergo,  have  been  already  mentioned  in  connection  with  the  develop- 
ment of  the  abdominal  viscera. 

Development  of  the  muscles. — The  muscles  of  the  trunk  are  formed  from 
the  protovertebrse.  These  are  at  first,  as  previously  described,  separate  masses  of 
mesoblast,  the  cells  of  which  have  at  the  periphery  of  the  mass  a  tendency  to  a 
radial  disposition  (fig.  189,  A),  whilst  toward  the  centre  they  are  loosely  arranged, 

'  The  manner  in  which  the  pleurae  are  invaginated  by  the  growing  lungs  has  already  been  alluded  to 
(p.  110). 


160 


DEVELOPMENT  OF  THE  MUSCLES. 


and  may  even  leave  a  more  or  less  distinct  space  unoccupied  l)y  cells  (protovertebral 
cavity).  This  cavity  has  occasionally  been  noticed  (Lockwooi,  Bonnet)  to  be 
continuous  laterally  with  the  mesoblastic  (coelomic)  cleavage  (fig.  189,  B  ;  see  also 


Fig.  189. — Two   SECTIONS   of   a    sheep 
EMBKYO.      (Bonnet.) 

A,  shows  the  cavity  within  the  pi'oto- 
vertebrte.  In  B,  the  protovertebra  on 
the  loft  side  of  the  section  is  united  with 
the  lateral  mesoblast ;  on  the  right  side  its 
cavityalsois  continuous \rith  the  ccelomic 
cleft  in  that  mesoblast.  am,  amnion  ; 
n.c,  neural  canal  ;  p.v,  protovertebra  ; 
ao,  aorta  ;  'jp.'p,  pleuro-peritoneal  space 
(ccelom). 

fig.  139,  p.  llTjj  and  it  is 
probably  the  morphological  equi- 
valent of  the  coelom  in  this  part 
of  the  mesoblast.  Wliether  there 
be  originally  a  cavity  or  not  in  it, 
the  protovertebra  presently  be- 
comes filled  up  with  cells  and 
then  forms  a  fairly  compact  mass 
of  cells  which  are  mostly  iiTegu- 
larly  aiTanged,  but  externally 
(next  to  the  cutaneous  epiblast) 
become  regularly  disposed  into 
an  epithelium-like  plate  of  co- 
lumnar cells.  This  is  known  as 
the  muscle  plafe,  and  when  the 
inner  part  of  the  protovertebra  becomes  broken  up  as  a  distinct  mass  and  joins 
with  the  neighbouring  protovertebrte  to  form  the  membranous  vertebral  column 
(see  below),  the  muscle  plates  still  remain  distinct  :  in  them  therefore  the  original 

Fig.    190. — Teans^'erse    section 

OF  THE  TRUXK  OF  A  CAT  EMBRYO, 
SHOWING    MUSCLE    PLATES. 

(E.  A.  S.) 

\  ?)z.p. ,  muscle  plate  :  ao,  aorta  ; 
m.g. ,  mid-gut ;  am,  amnion  ;  ■z';, 
ve.sicle  of  Wolffian  body  ;  w.d., 
Wolffian  duct. 


mesoblastic  segmentation 
continues  to  be  exhibited. 
They  do  not  long  remain 
as  a  single  epithelium-like 
layer,  for  the  extremities 
of  this  layer  fold  sharply 
round  and  become  continu- 
ous with  a  cell-stratum, 
Avhich  immediately  lines  the  interna,  surface  of  the  columnar  layer  and  forms  an 
i/ine)-  muscle-jjlate  (fig.  190).  It  is  uncertain  whether  the  cells  of  this  inner  muscle- 
plate  are  derived  from  part  of  the  columnar  layer  which  has  folded  over,  or  whether 
they  spring  from  other  cells  of  the  protovertebra.  Soon  after  their  appearance  as  a 
distinct  layer  of  the  muscle-plate  they  begin  to  elongate  in  the  sagittal  (antero- 


FORMATION    OF    THE    MUSCLKS    OF    TIIK    UKAI).  IGl 

posterior)  diroctittn  (liij;.  I'.'l,  /.///.),  and  it  may  pivst'iitly  be  observed  thuc  they  are 
becoming  developed  into  longitudinal  groups  or  segments  of  muscle-fibres  which 
stretch  between  the  original  intervals  between  the  protovertebrae.  The  destinatJDii 
of  the  onto?- layer  of  the  musele-iilate  has  not  been  traced  with  certainty.     Ikdluur 


^^aetf^^l^ibliljafetHjji:'  i,' J.  ■  t^iy^' 


Fig.  1!>1.  —  HoKI/.ONTAI,  I.O.NdlTl'DINAL 
SKCTION  OF  THKKK  I'lKiTOVKUTK- 
BR.K  IN  A  SNAKK    KMIIUYO. 

(v.  Ebner. ) 

tp,  cutaneous  epiblast  ;  e.  m,  exter- 
nal layer  of  luusdc-plate  ;  /,  its  mar- 
gins folJeil  round  into  l.hi,  internal 
layer  of  niuscic-jilate  coniiiosed  of 
flattened  cells  which  are  Ijccoiiiing 
elongated  into  muscular  fibres  ;  n.c, 
neural  civnal,  in  outline  only  ;  n'.c', 
neural  epiblast  forming  its  walls. 
Between  these  and  the  muscle-plate 
is  a  continuous    mcsoblastic    tissue 

which  has  been  derived  from  the  inner  ]iarts  of  the  protovei-tcbrrc,  partly  interrupted  by  the  ganglion 
ludiments,  ///.  The  original  intervals  between  the  protovcrteluaj  here  are  ;:till  indicated  by  vessels,  v. 
i.cl,  cleft  in  the  deeper  protovertebral  tissue  (according  to  Ebner  this  is  the  remains  of  the  original 
protovertebral  cavity). 

described  it  as  also  eventually  becoming  transformed  into  mnscle-libres  (in  elasmn- 
braiichs),  l)ut  others  have  failed  to  confirm  this  opinion,  and  it  is  by  some  believed 
that  it  may  assist  in  the  formation  of  the  cutis  vera. 

Although  the  muscle-plates  are  originally  mainly  concerned  with  the  foiination 
of  the  muscles  which  move  the  central  skeletal  axis,  it  is  probable  that  all  the 
skeletal  muscles  l)oth  of  the  trunk  and  limbs  are  eventually  derived  from  them  (see 
below,  p.  IGo). 

Formation    of   the    muscles   of  the    head    and   evidences  of  head    segmentation. — 

Althoui!'h  perhaps  no  part  of  the  cranium  actually  represents  a  vertebra,  there  is  nevertheless 
abundant  evidence  of  an  orig-inal  segmentation  of  the  head  corresponding-  with  the  mesoblastic 
somites  of  the  trunk.  Such  segmentation  is  shown  by  the  existence  of  the  visceral  arches, 
which  in  the  typical  and  least  modified  vertebrates  (c.//.,  elasmobranchs)  are  at  least  nine 
in  number,  by  the  successive  separat'on  of  the  part  of  the  body  cavity  which  extends  into 
the  head  into  separate  portions,  or  head  cavities,  one  corresponding'  to  each  visceral  arch,  the 
parietes  of  which  develop  into  muscles,  and  which  therefore  correspond  with  the  muscle- 
plates  of  the  protovertebral  of  the  trunk  (this  is  in  fact  the  typical  mode  of  formation  of 
mesoblastic  somites,  r.  page  26),  and  lastly  by  the  mode  of  development  of  the  cranial 
nerves  and  their  relations  to  the  visceral  and  branchial  arches,  which  correspond  in  a  general 
way  with  the  relations  of  the  ventral  branches  of  the  spinal  nerves  to  the  ribs. 

The  formation  and  destination  of  the  head  cavities  have  been  investigated  of  late  years 
(chiefly  in  elasmobranchs,  but  also  iu  reptiles  and  biixls)  by  Balfour.  Milnes  Mar.<hall.  and 
van  Wijhe,  and  the  result  of  these  investigations  tends  to  show  that,  in  all,  nine  portions  of 
the  original  head  cavity  become  separated  off  on  either  side,  their  formation  proceeding  from 
behind  forwards.  Each  somite  cavity  becomes  subsequently  divided  into  a  dorsal  part  corre- 
sponding with  the  protovertebrte  of  the  trunk  and  a  ventral  part,  corresponding  with  the 
pleuroperitoneal  cavity,  and  lying  in  the  middle  of  the  corresponding  visceral  arch  (■).  Both 
pai-ts  give  rise  by  differentiation  of  their  parietes  to  muscles  ;  the  visceral  arch  portions  to 
the  muscles  of  the  jaw  and  hyoidean  and  branchial  apparatus  ;  the  doj-sal  portions,  some  (first, 
second,  and  third)  to  the  muscles  which  move  the  eyes,  some  (seventh,  eighth,  and  ninth)  to 
the  muscles  which  connect  the  head  with  the  shoulder  gii-dle.  v.hilst  some,  viz.,  the  foui-th, 
fiith,  and  sixth,  are  said  to  disappear.  The  fii'st  head  cavity  forms  the  eye  muscles,  which  are 
supplied  by  the  third  nerve :  the  second,  the  muscle  supplied  by  the  fourth  (superior  oblique)  : 
and  the  third,  the  muscle  supplied  by  the  sixth  (external  rectus).  In  higher  vertebrates,  the 
formation  of  the  head  cavities  and  thau-  subsequent  destination  have  not  been  as  yet  clearly 
followed  out,  although  indications  of  their  existence  are  not  wanting. 

'  According  to  v.  Wijhe  the  intermediate  part  of  the  typical  somite  cavity  represents  a  segmental 
(uriniferous)  organ,  but  this  intermediate  part  is  not  scea  iw.  the  head,  although  it  begins  to  appear  in 
the  immediately  succeeding  somites. 

.    VOL.  I.  M 


163  DEVELOPMENT  OF  THE  VERTEBRAL  COLUMN. 

Development  of  tlie  vertebral  column. — The  vertebral  column  is  developed 
around  the  notochord,  except  at  the  anterior  end  of  that  structure,  which  is  imbedded 
in  the  basis  cranii.  It  is  formed  from  protovertebral  mesoblast.  The  outer  part  of 
each  protovertebra  is  transformed  into  a  muscle-plate,  and  thus  the  original  meso- 
blastic  segmentation  is  maintained.  The  inner  parts  of  the  protovertebrse  do  not, 
however,  remain  distinct,  but  blend  with  one  another  on  each  side  of  the  neural 
canal  to  form  a  longitudinal  mass,  which  extends  to  the  side  of  and  subsequently 
encloses  the  notochord,  and  finally  sends  dorsal  prolongations  over  the  neural  canal, 
so  that  this  also  receives  a  continuous  mesoblastic  investment,  forming  the  membrana 
reuniens  superior  of  Eemak.  The  investment  is  only  incomplete  opposite  the  points 
where  the  nerve  roots  are  connected  with  the  spinal  cord,  and  in  it  there  is  no  sign 
of  any  differentiation  into  vertebrae.  It  is  continuous  with  a  similar  investment 
within  the  cranium,  which  extends  in  front  of  the  notochord  into  the  fronto-nasal 
process.  The  mass  of  mesoblast,  which  thus  encloses  the  notochord  and  neural 
canal,  is  often  spoken  of  as  the  membranous  vertebral  column  and  cranium,  but  it 
represents  much  more  than  the  cartilaginous  and  bony  structures  of  those  parts,  all 
the  investing  membranes  of  the  cord  and  brain  and  the  ligaments  of  the  vertebrse 
being  also  derived  from  it.  From  it  septa  pass  between  the  muscle-plates  and  serve 
to  give  attachment  to  the  developing  muscle-fibres. 

The  first  appearance  of  the  permanent  vertebra  is  in  the  form  of  cartilage,  which 
becomes  formed  in  this  mesoblastic  investment  on  either  side  of  the  neural  canal, 
nearly  opposite  the  interval  between  each  two  muscle-plates,  to  form  the  neural 
arch.  This  part  of  the  vertebra  therefore  alternates  with  the  original  mesoblastic 
somites  as  represented  by  the  muscle-plates. 

According  to  Froriep,  the  lateral  halves  of  each  cartilaginous  neural  arch  become 
united  below  the  notochord  before  the  appearance  of  the  rudiments  of  the  cartilagi- 
nous bodies,  and  the  latter  appear  as  median  accumulations  of  cartilage,  immediately 
posterior  to  the  hypochordal  part  of  the  cartilaginous  arch.  In  most  of  the  vertebra 
this  hypochordal  part  of  the  arch  soon  disappears  as  a  distinct  structure,  but  in  the 
atlas  vertebra  the  primitive  condition  is  maintained. 

The  serial  arrangement  of  the  musculature  represents  phylogenetically  the  original 
segmentation  of  the  vertebrate  body.  The  segmentation  of  the  vertebral  column, 
on  the  other  hand,  has  been  arrived  at  later,  and  has  been  carried  out  in  depen- 
dence upon  the  muscular  segmentation.^ 

The  cartilage  makes  its  appearance  on  the  fom'th  day  in  the  chick,  on  the 
eleventh  or  twelfth  day  in  the  rabbit,  and  in  the  fom-th  or  fifth  week  in  man 
(Kolliker).  It  is  completed  by  the  sixth  or  seventh  week,  soon  after  which  ossifica- 
tion commences.  To  form  the  intervertebral  discs,  the  mesoblast  between  the 
bodies  of  the  vertebree  acquires  a  fibro-cartilaginous  character,  while  at  the  same 
time  the  notochord,  which  gradually  elsewhere  becomes  reduced  in  size  and  eventu- 
ally disappears,-  here  undergoes  enlargement,  and  its  cells  form  an  irregular  network 
in  the  central  intervertebral  pulp.  Its  remains  are  found  at  aU  periods  of  life  in 
the  middle  of  the  discs  (Luschka). 

In  the  osseous  fishes  there  is  an  intervertebral  dilatation  of  the  notochord,  the  growth  of 
which  proceeds  to  s-uch  a  considerable  extent  as  to  give  rise  to  a  mass  of  soft  gelatinous 
substance,  which  occupies  the  conical  hollows  of  the  biconcave  vertebral  bodies.  But  in 
birds,  reptiles,  and  amphibia,  where  synovial  articulations  are  developed  between  the  vertebral 
bodies,  the  notochord  soon  disappears  from  the  intervertebral  spaces,  although  its  remains  are 
seen  for  some  time  in  the  bodies  themselves  (Gegenbaur). 

In  mammals,  the  notochord  is  constricted  within  the  cartilaginous  vertebral  body,  but 
dilated  in  the  intermediate  parts  of  this  rod,  the  whole  cord  being  monilif  orm.  At  a  somewhat 
later  period  small  dilatations  are  also  to  be  seen  in  the  epiphysial  cartilages  (fig.  103). 

1  A.  Froriep,  "  Zur  Entwickl.  der  Wirbelsaule,  8z;c.,"  Archiv  f.  Anatomie    1883  and  1S86, 


THE    LIMBS^. 


1G3 


Ribs  and  Sternum. — The  ribs  are  foi-ined  l)y  separate  cartilau^inous  traiisfor- 
niarioii  ill  fxicnsimis  dl"  the  protoverteliral  luesohlast  l)et\veen  the  muselc-phites. 
Aceordiiiy  to  some,  they  grow  out  from  tlie  cartilaginous  vertelirae,  Init  become  separate 
before  ossification  begins.  Simihir  deposits  are  formed  in  connexion  with  the  other 
vertebrte  (except  the  coccygeal  in  man),  but  they  here  become  united  by  ossification 
with  and  fonu  parts  of  tiie  vertebrae  (see  Osteology,  Vol.  11.).     At  their  \entral 


Fig.  192. — Sections  op  the  vertebral  column  op  a  Hrr-iAN  fcetus  op  eight  weeks. 

(From  Kolliker. ) 

A,  transverse  longitudinal  section  of  several  vertebrne.  1,  1,  clionla  dorsalis,  its  remains  tliicker 
opposite  the  intervertebral  discs  ;  2,  is  placed  on  one  of  the  bodies  of  the  permanent  vertcbne  ;  3,  on 
one  of  the  intervertebral  discs. 

B,  transverse  horizontal  section  through  a  part  of  one  dorsal  vertebra.  1,  remains  of  the  chorda 
doi-salis  in  the  middle  of  the  body  ;  2,  arch  of  the  vertebra  ;  3,  head  of  a  rib. 

Fig.  193. — Sagittal  section  of  a  dorsal  intervertebral  ligament  of  an  advanced 
sheep's  embryo.     (Kolliker.) 

La,  l.p,  anterior  and  posterior  ligaments  ;  l.i,  intervertebral  ligament  ;  Ic,  Jc',  cartilaginous  ends  of 
two  vertebral  bodies,  w,  w' ;  c,  enlargement  of  notochord  in  the  ligament  ;  c',  c",  enlargements  in  the 
cartilaginous  ends  of  the  vertebrae. 

extremities  the  first  seven  (thoracic)  cartilaginous  ribs  become  united  on  either  side 
into  a  longitudinal  cartilaginous  plate,  and  this  afterwards  joins  its  fellow  of  the 
opposite  side  to  form  the  sternum  (nianubrimu  and  body).  The  xiphoid  i.s  of  later 
formation  (Parker).  This  mode  of  development  of  the  sternum  explains  many  of 
the  malformations  in  the  shape  of  fissures  of  the  sternum  of  different  gi'adation 
which  have  been  observed. 

TTie  Limbs. — The  limbs  arise  as  outgrowths  from  the  lateral  part  of  the 
trunk  in  the  thoracic  and  pelvic  regions  in  the  third  day  in  the  chick  and  in  the 
third  and  fourth  week  in  the  human  embryo.  They  appear  as  flattened  semilunar 
thickenings  of  the  parietal  mesoblast  covered  by  epiblast,  budding  out  from  a  lateral 
ridge  which  is  seen  in  the  early  embryo  near  the  line  of  cleavage  of  the  mesoblast 
and  close  to  the  outer  margins  of  the  muscle-plates,  and  several  of  which  subsequently 
send  prolongations  into  each  liml)  (i)  ;  they  are  therefore  connected  with  several 
mesoblastic  somites,  as  is  also  indicated  by  their  nerve  supply. 

*  This   is   the  case  in  elasmobvanchs   (see  fig.    194,  from   Balfour),    but,  according   to    Paterson. 


164 


'rSE   LIMBS. 


Each  limb  consists  of  a  part  which  is  sunk  in  the  substance  of  the  lateral  ridge, 
and  in  ^Yhich  the  thoracic  or  pelvic  girdle  becomes  developed,  and  of  a  free  or 


sp-c 


•vbtnc, 


Fig.  194. — Tkaxsverse  sectiox  theoitgh  an  anterior  part  of  the  trunk  of  an  embryo  of 

ScrLLirii.     (Balfour.; 

si^.c,  spinal  cord;  sp.^;  ganglion  of  posterior  root;  ar,  anterior  root;  dn,  dorsal;  s^;.  ;*.  ventral 
branch  of  spinal  nerve  ;  mp,  muscle  plate ;  m2j,  part  of  muscle  plate  already  converted  into  muscle  ; 
mjy.l;  part  of  muscle  plate  extending  into  the  limb ;  nl,  nei-vus  lateralis ;  ao,  aorta  ;  ch,  notochord ; 
sy-ff,  sympathetic  ganglion;  ca.v,  cardinal  vein;  sr2,  segmental  duct;  st,  segmental  tube;  du,  duode- 
num; hji.d,  junction  of  hepatic  duct  with  it;  pan,  rudiment  of  pancreas  connected  with  another  part 
of  duodenum  ;  v./itc.  opening  of  iimbilical  canal  (vitelline  duct). 


projecting  part  (fig.  195).  This  is  at  first  quite  simple,  and  represents  the  distal 
segment  of  the  limb  (hand  or  foot).  The  other  two  segments  (forearm  and  leg; 
arm  and  thigh)  are  successively  marked  off  between  it  and  the  girdle  by  the 
development  of  transverse  furrows  representing  the  joints  (fifth  and  sixth  week). 
At  about  the  same  time  four  notches  appear  in  the  flattened  distal  extremity, 
marking  off  the  Intervals  between  the  fingers  and  toes,  and  the  middle  segment 
(fore-arm   or  leg)   begins   to   be   flexed   upon   the  proximal  (arm  or  thigh),  the 

the  limbs  do  not  receive  such  prolongations  fi-om  the  muscle-plates  in  bii-ds  and  mammals  ;  the  mr.scles 
develop  in  situ  from  previously  indifferent  mesoblast. 


THE   ClJANIUxM. 


1G5 


Concavity  looking  forwards  in  tiie  upper  limli  and  Iiackwards  in  the  lower  limb. 
The  liinljs  also  come  to  he  folded  ventrally  against  the  hody  of  the  enil>ryo. 

From  tiie  manner  in  whieh  the  flattened  limh-hud  grows  (ut  from  the  lateral 
ridge,  it  is  obvious  that  its  surl'aces  must  at  lirst  be  dorsal  and  ventral.  The 
dorsal  surface  afterwards  becomes  extensor  and  the  ventral  flexor.  The  anterior 
edge  is  respectively  the  radial  and  tibial ;  the  posterior,  the  ulnar  and  filtular.     Xa 

Fig.     19r>.  —  OlTLIXKS       <iK       TllK       A.NTKlili'U       KXTIIKJUTIKS 
OK     niMAS      EMUUYOS      AT       IiIFKKUK.NT      A«ES.         (After 

His.) 

>/,  at  four  weeks;  /?,  at  five  weeks  ;  C,  at  seven  weeks  ; 
D,  at  nine  or  ten  weeks. 

development  proceeds,  a  half  rotation  occurs  in 
opposite  directions  in  the  two  limbs,  resulting 
in  the  middle  flexure  (elbow,  knee)  being  directed 
forwards  in  the  ujiper,  backwards  in  the  lower 
limb. 

The  bones  of  the  limbs  are  laid  down  as 
cartilages  which  ajjpear  as  separate  differentia- 
tions of   the  more  centrally   placed  mesoblast, 

a  portion  of  mesoblast  remaining  for  a  time  undifferentiated  opposite  each  synovial 
articulation.  "Within  this  a  cleft  subsequently  appears,  and  enlarges  to  form  the 
synovial  cavity,  the  mesoblast  which  bounds  the  cleft  developing  eventually  into 
the  synovial  membrane  and  capsular  ligaments  of  the  joint. 

The  cranitira. — In  the  head  the  notochord  extends  as  far  forwards  as  the 
mid-brain.  Here  also  it  is  invested  by  a  continuous  mass  of  mesoblast,  which  sends 
lateral  prolongations  over  the  neural  canal  as  in  the  trunk  (membrana  reuniens). 


^ 


:Et/t. 


Fig.  196. — Diagrams  of  the  cartilagixocs  craxium.     (Wiedei-sbeim.) 

A,  First  stage. 
C7(,  notochord  ;  Tr,  trabeculse  cranii  ;  P.ch,  parachordal  cartilages  ;  P,  situation  of  pituitarj'  body-; 
A',  E,  0,  situations  of  olfactory,  visual  and  auditory  organs. 

B,  Second  stage. 
B,  basilar  cartilage  (investing  mass  of  Eatbke)  ;  .9,  nasal  septum  and  ethmoidal  cartilage  ;  Etli, 
Eth',  prolongations  of  ethmoidal  around  olfactory  organ,  completing  the  nasal  capsule  ;  01,  foramina 
for  passage  of  olfactory  nerve-fibres  ;  N,  E,  0,  C/i,  Tr,  as  before. 

The  main  difference  in  development  between  the  cranium  and  vertebral  column 
consists  in  the  fact  that  no  separate  cartilaginous  deposits  to  form  vertebrae  occur 


166         FORMATION    OF   THE    VISCEEAL    SKELETON    OF    THE    HEAD. 

withiu  the  head,  nor  caa  any  parts  be  distinguished  which  strictly  represent  vertebrse. 
The  cartilage  of  the  basis  cranii  makes  its  appearance  in  the  form  of  two  longitudinal 
bars  lying  on  either  side  of  the  notochord  {paracJiordal  cartilages),  and  of  two  other 
bars  (fraieculce  cranii  of  Rathke)  which  embi'ace  the  pituitary  body,  and  which 
become  united  together  in  front  and  with  the  parachordals  behind  to  form  a  con- 
tinuous mass,  which  posteriorly  completely  invests  the  notochord  (fig.  196).  The 
cartilaginous  basis  cranii  may  therefore  be  distinguished  into  the  parachordal  and 
prechordal  parts.  Of  these  the  first  represents  the  basi-occipital  and  basi-sphenoid, 
the  second  the  presphenoid  and  ethmoid  portions.  From  the  basis  cranii  continuous 
cartilaginous  plates  grow  on  either  side  over  the  cerebral  vesicles  to  a  greater  or 
less  extent  in  different  animals,  least  in  mammals,  where  only  the  occipital  region 
becomes  thus  roofed  in  by  cartilage.  Anteriorly  the  united  trabeculse  cranii  stretch 
forwards  into  the  fronto-nasal  process,  where  they  form  the  ethmoid  cartilage  and 
nasal  septum,  besides  enclosing  the  nasal  pits  Q^t^LWoWj  {cartilaginous  nasal  capsule). 
From  the  sides  of  the  presphenoid  portion  the  orbito-sphenoids  (lesser  wings), 
containing  the  optic  foramina,  are  developed,  and  from  the  sides  of  the  basi-sphenoid, 

Fig.  197. — View  frou  below  of  the  cartilagixous 

CBASIiJJI     WITH     ITS      OSSIFIC      CENTRES     IX     A     HUMAX 

FffiTcs  OF  ABOUT  FOUR  3I0STHS.     (After  Huxley.) 

The  cartilage  is  dotted  to  distinguish  it  from  the  bone 
which  is  shaded  with  lines. 

h.o,  basi-occipital  ;  a.o,  lateral  occipitals  ;  f.ni,  foramen 
/"                       -      .X  magnum;  o.c,  o'.c',  bony  deposits  in  the  periotic  capsule  ; 

(--  '                     v;,c   *  '  '      ~~-5  2'-*%  post-sphenoid  ;  jpr.s,  pre-sfihenoid  ;    o.s,  orbito-sphe- 

^ ^ — '    ;        '  ', noid  ;  s.n,  septum  nasi, 

■  \  '   '     ■■    [        (  '    '• 

{    5 J       -    r...  j       :__^  '■  the  greater  wings  or  alisphenoids.     A  cartilagi- 

*i,T:?r.\     ,  - :  J;.  ■*  j,          nous  capsule,  connected  with  the  parachordal 

y'    ' .:         ,,      Li,.^  -        ■    /        portion  of  the  basis  cranii,  invests  the  otic  vesicle 

{periotic  capsule).     TViDhin  this,  bony  centres  are 

eventually   formed,   which    unite   to  form   the 

petro-mastoid.     In  the  human  embryo,  chondri- 

fication  begins  in  the   fom'th  or  fifth  week  in 

the    basilar    portion    of    the    skull,    and    is    nearly    completed    by    the    eighth 

week. 

Formation  of  tlie  visceral  skeleton  of  the  head. :  cartilaginous  bars  of 

the  visceral  arches. — A  cartilaginous  bar  extends  from  the  periotic  capsule  and 

basis  cranii,  within  each  of  the  first  three  visceral  arches,  and  passes  forwards  to 

meet  its  fellow  in  the  middle  line.     The  har  of  the  mcmclibv.lar  arch  is  known  as 

3IecJceVs  cartilage.     It  is  visible  in  all  sections  of  the  foetal  jaw  up  to  the  seventh 

month.     Its  proximal  end  is  attached  at  first  to  the  basis  cranii,  afterwards  to  the 

periotic  capsule;  its  distal  end  joins  that  of  its  fellow  in  the  middle  line  of  the 

lower  jaw.     Only  near  this  conjoined  part  does  Meckel's  cartilage  take  part  in  the 

formation  of  the  lower  jaw  bone,  the  greater  part  of  this  bone  being  developed  by 

ossification  at  several  places  in  the  connective  tissue  around  the  cartilage.     In  some 

animals  a  short  cartilaginous  bar  is  formed  in  the  maxillary  process  {palato-pterggoid 

har,  fig.  198,  K,ppg).     Close  to  it  the  palatine  and  pterygoid  bones  are  formed  in 

membrane,  but  the  bar  itself  entirely  disappears.     The  second  or  hyoid  bar  arises 

from  the  skull  close  behind  the  attachment  of  Meckel's  cartilage,  and  passes  along 

the  second  arch.     It  disappears  in  part,  but  in  part  is  converted  into  the  styloid 

process,  stylo-hyoid  ligament,  and  lesser  cornu  of  the  hyoid  bone.     The  body  of  the 

hyoid  bone  (basi-hyal)  is  an  intermediate  formation  between  the  second  and  thu'd 

arches.     The  bar  of  the  third  arch  is  known  as  the  ihyro-hyoid.     Its  lower  end  forms 

the  greater  cornu  of  the  hyoid  bone  ;  bu*'-  its  attachment  to  the  skull  early  disap- 


FORMATION    OF    THE   AUDITORY    OSSICLES. 


10? 


eiA 


]i.y    wj 


jia.ck        nc 


Fig.  198,  A. — Elements  of  tiie  skull  of  an  embryo  pig,  ?,-incii  lonc,  viewed  from  below  : 
sEMi-i>iAGRAMMATic.     (From  Baifour  after  Parker.) 

pa.ch,  parachordals  ;  nc,  notot-horcl  ;  ait,  otic  capsule  ;  pn,  pituitary  ;  tr,  trabecule  ;  c.tr,  cornu 
of  tlie  trahecula  ;  pn,  prenasal  cartilage;  c.n,  external  uaies  ;  ol,  olfactory  region  ;  7>.;>^,  ijalato- 
pterygoid  bar  enclosed  in  the  luaxillo-pterygoid  process  ;  run,  mandibular  bar  ;  hy,  hyoidean  bar  :  th.h, 
thyrohyoid  bar ;  la,  aperture  for  facial  nerve  ;  8rt,  for  glossopharyngeal ;  86,  for  vagus  ;  9,  for  hypoglossal. 

Fig.  198,  I).— Side  view  of  the  siandibilar  and  hyoid  arches  in  an  embryo  pig  of  1^  inch 
,  in  length.     (From  Balfour,  after  Parker. ) 

tg,  tongue  ;  ink,  Meckel's  cartilage  ;  ml,  body  of  malleus  ;  inh,  its  manubrium  or  handle  ;  t.tii, 
tegmen  tyuipaui  ;  /,  incus  ;  M,  stapes  ;  i.h;i,  inter-hyal  ligament  ;  st.h,  stylo-hyal  cartilage  ;  h.h, 
hypohyal  ;  bh    basibranchial ;  th.h,  rudiment  of  first  branchial  arch  ;  7a,  facial  nerve. 


£?^2*f?; 


X  •'  c-ct/^tilaxr&^ 


Jiyo> 
cctrtttcujfc: 


shofi 


in.cvLS    T'^^^^^^^^     Meckel's  oca-Ula^e. 


7i.U'jz<Z  cccrtiZccae' 


■^uli 


'nioUiccLs 


Meckel  (xxt-itla^ey 


ccofttlct(J&  '-^^iSs^ 

fig.  1&9. prepi.rations  showing  three  stages  of   the 

clrtilaginous  bars   of   the  1st  and   2nd  arches  in 
THE  embryo  uf  THE  SHEEP.     (Salcnsky. ) 


pears,  or  in  some  animals  may  not 
be  found  at  all.  This  bar  is  in 
all  cases  of  less  importance  than 
the  first  two,  from  the  proximal 
parts  of  which  important  struc- 
tures connected  in  lower  verte- 
brates with  the  suspensory  appa- 
ratus of  the  lower  jaw,  in  the 
higher  vertebrates  with  the  ap- 
paratus for  sound  transmission 
to  the  internal  ear  are  developed. 
Formation  of  the  auditory 
ossicles. — If  the  embryonic  de- 
velopment of  the  bars  is  studied 
in  man  and  other  mammals,  it  is 
found  that  at  a  certain  period  of 
fcetal  life  Meckel's  cartilage  is 
directly  continuous  with  the  car- 
tilaginous malleus,  above  which, 
and  at  first  in  direct  continuity 
with  it,  is  the  cartilaginous  incus 
(fig.  199)  ;  these  two  ossicles 
therefore  appear  as  the  enlarged 
and  modified  proximal  end  of 
Meckel's  cartilage.  Somewhat 
later  the  incus  becomes  detached 
from  the  malleus,  but  the  latter 
long  remains  in  continuity  with 
Meckel's  cartilage  (fig.  iOCi). 


168 


FORMATION    OF    THE    AUDITORY    OSSICLES. 


The  most  obvious  interpretation  of  these  appearances  would  seem  to  be  that  both  the 
malleus  and  incus  are  developed  from  the  cartilaginous  bar  of  the  first  or  mandibular  arch. 
This  view  is  not,  however,  universally  held.  For,  as  the  several  illustrations  show,  the  second 
or  nyoid  bar  is  also  connected  to  the  developing  incus,  through  which  it  is  joined  to  the  periotic 
capsule.  Hence  it  has  been  inferred  by  some  that  the  incus  belongs  to  the  hyoid  bar,  and  not 
to  the  mandibular  (A.  Eraser).  Others  have  looked  upon  it  as  representing  a  hyomandibular 
cartilage,  like  that  which  in  Sauropsida  forms  a  common  suspensory  apparatus  for  both 
mandibular  and  hyoidean  apparatus  (Huxley).     This  hyomandibula  itself,  however,  may  repre- 


Fig.  200. — Condition  of  Meckel's  car- 
tilage AND  THE  HYOID  BAR  IN  THE 
HUMAN    FCETUS    OF    ABOUT  18   WEEKS, 

(KoUiker. ) 

B,  is  an  enlarged  sketch  by  Allen 
Thomson,  showing  the  relationship  of 
the  several  parts  better  than  in  A. 

z,  zygomatic  arch  ;  ma,  mastoid  pro- 
cess ;  Ttii,  portions  of  the  lower  jaw  left 
in  situ,  the  rest  having  been  cut  away  ; 
M,  Meckel's  cartilage  of  the  right  side, 
continued  at  s,  the  symphysis,  into  that 
of  the  left  side  31' ,  of  which  only  a  small 
part  is  shown  ;  T,  tympanic  ring  ;  m, 
malleus  ;  i,  incus  ;  s,  stapes  ;  sta,  stape- 
dius ;  st,  styloid  process  ;  p,  h,  g,  stylo- 
pharyngeus,  stylohyoid,  and  styloglossus 
muscles ;  stl,  stylohyoid  ligament  attached 
to  the  lesser  cornuof  the  hyoid  bone,  hy; 
ill,  thyroid  cartilage. 

sent  an  upward  prolongation  of 
Meckel's  cartilage,  and  would  then 
belong  to  the  1st  visceral  arch 
(Peters).  Lastly,  yet  another  solu- 
tion of  the  question  has  been  offered, 
viz.,  that  both  incus  and  malleus  are 
hyomandibular  (Albrecht,  G-adow). 

The  last  mentioned  opinion  is  based 
mainly  ujoon  considerations  of  com- 
parative anatomy,  which  can  hardly 
be  left  out  of  account  in  dealing  with 
the  morphology  of  these  structures. 
In  lower  Vertebrata  the  suspensory 
apparatus  of  the  lower  jaw  comprises 
besides  the  hyomandibula,  common 
to  it  and  to  the  hyoid  apparatus,  a 
large  bone,  known  as  the  quadrate, 
by  means  of  which,  either  directly  or 
with  the  intercalation  of  an  os  arti- 
cular e,  the  lower  jaw  is  united  with 
the  basis  cranii  and  periotic  capsule. 
Reichert  looked  upon  the  incus  as  the 
homologue  of  the  quadrate,  and  the  malleus  as  that  of  the  os  articulare  ;  and  the  same  view 
was  taken  by  Gegenbaur.  Huxley,  on  the  other  hand,  came  to  the  conclusion  that  the 
homologue  of  the  quadrate  bone  of  reptiles  and  birds  is  to  be  found  in  the  malleus,  and  that 
the  incus  represents  a  portion  of  the  hyomandibular  bar,  which,  as  above  stated,  is  common  to 
both  first  and  second  arches.  Various  other  observers  have  concluded  that  the  quadrate  of 
lower  vertebrates  is  represented  in  mammals  by  the  zygomatic  process  of  the  squamosal. 
Gadow,  however,  looks  upon  it  as  represented  by  the  tympanic  ring  of  mammals. 

The  stapes  has  been  variously  described  as  representing  :  1 ,  a  part  of  the  hyoid  arch 
(Reichert) ;  2,  a  part  of  the  periotic  capsule  which  has  become  detached  (Parker) ;  3,  in  part 
or  wholly,  the  hyomandibula  of  lower  vertebrates  (G-egenbaur,  Huxley,  Albrecht,  G-adow) ; 
i,  hyomandibula  and  detached  periotic  cartilage  conjoined  (Gradsnigo)  ;  5,  as  an  independent 
circular  deposit  of  cartilage  around  the  stapedial  artery  ^  (Salensky,  Fraser).  It  is  at  any  rate 
closely  connected  with  the  hyoid  bar,  which  forms  from  above  clown  the  tympano-hyal  and 


This  artery  disappears  in  man,  but  is  persistent  in  many  mammals. 


RECENT    LITERATURE.  169 

styloid  processes,  the  stylo-hyoid  lijjainont  ami  iho  lessor  cunma  of  iIr-  liyoiil   l)oiic  (curato- 

The  reiuaiiiinjf  bones  of  the  visceral  skeleton  of  tlu;  lioad.  viz..  tlie  nia.xi'lary,  malar, 
jialatiue,  ptcry^oiil,  vomer,  nasal,  and  laohrj'mal,  are  all  formed  in  membrane.  An  account  of 
their  dcvulopmuut  is  given  in  the  Usteoloyy  (Vol.  II.). 


RECENT    LITERATURE. 

Ahlborn,  Ucbcr  die  Segmentation  des  W irhiUhii  rklirpers,  Zcitschr.  f.  wiss.  Zool.,  xl. 

Albrecht,  .S'lo-  la  ralcnr  morpkolor/ifjuc  dc  l' articulation  mandihulairc,  du  cartilage  de  Mrel-eXtt 
des  ossihts  de  Vuuie,  Bruxelles,  1883  ;  Sur  la  vahur  morpholoyique  da  la  trompe  d  Eustackv,  dkc, 
HruxuUes,  1884. 

Balfour,  On  the  development  of  the  skeleton  of  the  pai reel  fins  of  Elasmohranchii,  dc.,  Proc.  Zool. 
Soc,  18S1. 

Baiir,  (hi  the  quadrate  in  the  Mammalia,  Quarterly  Journal  of  Microsc.  Science,  August,  1887. 

Bemmelen,  v.,  Ueber  die  llerkunft  dcr  Extrcmitdten  u.  Zungenmusc-idatur  bei  Eidedisen,  Anal. 
Anzeiger,  lS8i». 

Braun,  M.,  l/eber  den  Schivanz  bei  Sdugcthieremhryonen,  Deutsche  Zeitsclir.  f.  Thicrmedicin,  ix., 
ISS:?. 

Cadiat,  Du  dtveloppimcnt  de  la partic  ccphalo-thoracique  de  Vimbryon,  ele  la  formation  du  dia- 
I'liruyma,  etc.,  Journal  de  I'anatomie,  T.  xiv. ,  1S78. 

Dohrn,  Bcmerkumjcn  ii.  d.  niucsten  Versueh  einer  Losung  eles  Wirbilthierkopf- Problems,  Anat. 
Auzeiuer.  1890.     Also  numerous  papers  in  Mitthcihingen  el.  znol.  Station  zu  A\a2)cL 

Dollo,  Oil  the  luallcus  of  the  Lacertdia  and  the  malar  and  quadrate  bones  of  Mammed ia,  Quarterly 
Juurnal  of  ?iIicrosc.  Science,  1883. 

Ebuer,  V.  v.,  Urwirbd  und  Neugliedcrung  dcr  Wirbdisiiule,  Sitzungsb.  der  Wiener  Akadein., 
1888. 

Fraser,  On  the  development  of  the  ossicuUi  audilus  in  the  higher  Mammalia,  Phil.  Trans.,  1882. 

Froriep,  A..,  Zur  Entieicklungsgcschichte  dcr  Wirhelsdule,  insbesondcre  des  Atlas  und  Epistro- 
phiu.-<  und  d(r  Occipitalngion,  Arch.  f.  Anat.  u.  Entwicklung.sgesch.,  1883  and  1886. 

Gadow,  On  the  modifications  of  the  first  and  second  visceral  arches,  with  especial  reference  to  the 
homolo'iiis  of  the  auditory  ossicles,  Philos.  Trans.,  1888. 

Geg-enbaur,  Die  Metamerie  des  Kopfes  und  die  IVirbcUhiorie  des  Kopfskelets,  Morphol.  Jahrb., 
xiii.,  1887. 

Gradenig-o,  G.,  Die  embryonale  Anlage  des  Mittelohres :  die  morphologische  Bidiutung  der 
Gehorknochelchen,  Wiener  med.  Jahrbiicher,  1887. 

His,  W.,  Mittheilungen  zur  Embryologie  der  Sdugethiere  u.  des  Menschen,  Aicbiv  f.  Anat.  u. 
Physiol.,  Anat.  Abth.,  1881. 

HofiEmann,  C.  K.,  Ue.  die  morphologische  Bedeutung  des  Gehorknochelchens  bei  d.  Reptilien, 
Zool.  Anzeiger,  xii. 

Kostlin,  Der  Bau  des  knochemen  Kopfes  in  den  vier  Klasse7i  der  Wirbellhiere,  Stuttgart, 
1884. 

Lockwood,  C.  B.,  The  early  development  of  the  pencardium,  diaphragm,  and  great  veins. 
Proceed,  of  the  Royal  Society,  1887. 

Noorden,  W.  v.,  Beitrag  zur  Anatomic  der  knoi-peligen  Schddclbasis  menschlirher  Embryonen, 
Arch.  f.  Anat.  u.  Phys.,  Anat.  Abth.,  1887. 

Parker,  W.  K.,  Various  important  papers  on  the  structure  and  development  of  the  skull  in  the 
Philosophical  Transactions  of  the  Royal  Society  and  the  Transactions  of  the  Zoological  Society. 

Rabl,  C,  Theorie  des  Mesoderms,  Morphol.  Jahrb.,  xv. 

Ravn,  E.,  Ue.  die  Bildung  dcr  Schcidcwand  zwischcn  Brust-  und  Bauch-hohle  in  Sdugethier 
embryonen.  Arch.  f.  Anat.  u.  Physiol.,  Anat.  Abth.,  1889  ;  Vntersuehungen  ue.  d.  Entivickl.  des 
Diaphragmas,  <i;c..  Arch.  f.  Anat.  u.  Physiol.,  Anat.  Abth.,  1889. 

Salensky,  Beitrdge  zur  Entwicklungsgeschichte  der  knorpeligen  Gehorknochelchen  bei  Sdugethicren, 
Morphol.  Jahrb.,  vi. 

Strahl  u.  Carius,  Beitr.  zur  Entwickl.  des  Herzens  u.  d.  KorperJiohlcn,  Arch.  f.  Anat.  u. 
Physiol.,  Auat.  Abth.,  1889. 

Strazza,  G.,  Zur  Lehre  uber  die  Entwicklung  der  Keldkopfmuskeln,  Wiener  medic.  Jahrbiicher, ' 


TJskow,  N.,  Ueber  die  Entwicklung  des  Zwcrchfclls,  des  Pcricardiums  und  des  Coloms,  Archie 
f.  luikrosk.  Anatomic,  Bd.  22,  1883. 

Wijhe,  J.  W.  van,  Ueber  die  Kopfsegmcnte  und  die  Phylogenic  des  Geruchsorganes  dcr  Wirbd- 
thieri,  Zoolog.  Anzeiger,  1SS6  ;  Die  Kopfregion  der  Cranioien  beim  Amphioxus,  Anat.  Anzeiger,  iv. 


tul..    I. 


170 


OUTLINES    OF    EAKLY    HUMAN    EMBRYOS. 


The   following   outlines   and   measurements   will   be   found  useful    to  assist  in 
determining  the  ages  of  early  human  embryos  : — 


12  Z  A 

(about  10  to  l:j  days)  (15  days)  (21  days) 

-inim.  2*5ram.  2'6niiu.  4mm. 


5 

6 

(27  days) 

(:J0  dajs 

7"5mm. 

'Jmm. 

i<@,^^\\ 


(34  days) 
10'5mm. 


(6  weeks) 
15'5mm. 


(7A  weeks) 
18"5mm. 


Fig.  201. — Outlines  of  human  embryos  of  vakious   ages   from    10  dats  to  7h  weeks.     (From 

Kollmann,  after  His.) 

The  representations  are  magnified  about  3  diameters.  The  actual  measurement  of  the  embryos 
is  given  below  each  one  :  it  is  taken  vei'tically.  In  4,  5,  6  and  7  it  does  not  therefore  include 
the  head. 


INDEX    OF    PART    I. 


Abdominal  stalk,  46 
Accessory  thyroids,  1 1 1 
Alee  nasi,  96 

Albrecht  on  anditory  ossicles,  168 
Alecithal  ova  (a,  not,  \-IJKvdoi,   oil-bottle),  defi- 
nition, 9 

invagination  of,  22 
Alimentary  canal,  development  of,  99 
Allantoic  arteries.     See  U.mdilical. 

Huid,  46 

tube,  36 
Allantois  (aAAny,  sausage,  uSos,  form),  formation 
of,  43 

first  appearance  of,  44 

structiu'e  01",  44 

in  what  animals  fonnd.  45 

hypoblastic  sac  of,  40 

pedicle  of,  46 

variation  of  period  of  development,  46 

connection,  with  alimentary  canal,  loi,  n. 
with  bladder,  122 
Amnion  (afivioy,  a  bowl  in  which  victim's  blood 
was   caught,  the  caul),    earliest  appear- 
ance of  in  human  embryo,  46 

false,  42 

formation  of,  42 

in  guinea  pig,  43,  46 

resulting  from  invagination,  24 

true,  42 
Amniota,  42 
Amphibia,  liver  in,  112 

Wolffian  duct  in,  iiS   . 

heart  in,  136 
Amphioxus  {au<pi,  double,    o^vs,  a  point),  ale- 
cithal ova  of,  9 

invagination  of  blastoderm  of,  22,  24 

origin  of  mesoblast  in,  22,  26 

niesenchjTne  in,  27 

notochord  in,  34 

mesoblastic  somites  in,  37 
Amygdalre  (oyu^SaATj,  almond},  66 
Anal  membrane,  the,  loS 

Analogy,   di/o,   according  to,  Aoyos,  ratio),  defi- 
nition of,  4 

examples  of,  4 
Anatomy,  definition  of,  i 

departments  of,  i 
Angelucci  on  zonule  of  Zinn,  S7 
Annelida  {anndlus,  little  ring),  2 
Anterior  perforated  space,  79 
Anus,  formation  of,  108,  12S 
Aorta  {aopTT],  aeipw,  to  lift),  inimitive,  formation 
of,  38 

descending  primitive,  146 

arch  of,  148,  150 

bulb  of,  144,  147,  148 
descending  permanent,  150 
Aortic  arches.     See  Aetekial  Arches. 


Acjueduct  of  Sylvius,  62,  67 

Aichenteric  cavity,  primitive  {apxv,  beginning, 

ivrepov,  intestine),  26 
Archiblastic  cells  of  His,  25 
-Ajteria  centralis  retina?,  85,  87 
Arterial  (aoitic)  arches,  formation  of,  38,  146 

division  of  aortic  septum  by,  144 

origin  of  principal  arteries  from,    146,  147, 
148 

destination  of  fourth  and  fifth,  150 
Arteries  {dpTi)pia,  afpu,  to  lift),  development  of, 
146 

from  arterial  arches,  148,  149 

from  arches  in  birds,  150 

ascending  phar}iigeal,  148 

auricular,  148 

basilar,  149 

carotid,  148,  150 

common  iliacs,  147 

innominate,  150 

intercostal,  149 

inter-segmental,  149 

lingual,  148 

maxillary,  148 

middle  sacral,  147 

occipital,  148 

pulmonary,  148,  150 

subclavian,  150 

temporal,  148 

umbilical,  44,  146,  155 

vertebral,  149 

vitelline,  40 
Arthropoda  {&pdpov,  a  joint,  ttovs,   a  foot),  seg- 
mental plan  of,  2 

ceuti'olecithal  ova  of,  8 
Ary epiglottic  folds,  103 

Arytenoid  cartilages  [apinawa,  a  pitcher),  103 
Ascaris  megalocephala,  polar  globules  observed 
in,  9 

fertilisation  in,  11,  12 

segmentation  of,  16 
Auditory  pit,  89 

vesicle,  89 

nerve,  78,  89,  93 

ossicles,  167,  168 
Auricular  canal,  142 
Axis,  primitive  skeletal,  2 


Baer,  vox,  zona  pellucida  of,  6 

on  bilaniination  of  middle   layer  of  blas- 
toderm, 23 
Balfour,  F.  M.,  on  amnion  and  allantois,  45,  n. 

blastodermic  layers,  nomenclature  of,  23 

coelom,  tj'pical  development  of,  24 

head  cavities,  161 

limbs,  163  n. 

lens-capsule,  87 


11 


IXDEX    OF    PART    I. 


Balfour,  F.  M. — continued. 

Miillerian  duct,  122,  123 

muscle  plate,  outer,  161 

nerves,  epiblastic,  origin  of,  57 
spinal,  anterior  roots  of,  74 

neural  crest,  73 

oesophagus,  obliteration  of  lumen  of,  104,  n. 

ova,  formation  of,  124 

parablast,  27,  n. 

primitive  groove,  23 

polar  globules,  tlieory  of,  14 

suprarenals,  122 

sympathetic  sj'stem.  Si 

Wolffian  body,  115,  n. 
duct,  118 

vitelline  membrane,  8 
Bars,  cartilaginous,  of  mandibular  arch,  166,  168 

palato-pterj'goid,  166 

hyoid,  166,  168 

thyro-hyoid,  166 
Bartholin,  glands  of,  128 
Basal  layer  of  placental  decidua,  5 1 

attachment  of  villi  to,  52 
Basis  cranii,  166 
Bat,  placental  sinuses  in,  52 
Bauchstiel  (abdominal  stalk),  46 
Beard  on  branchial  sense-organs,  76 
Beneden,  v.,  vitelline  membrane,  8 

formation  of  polar  globules,  9 

entrance  of  spermatozoon  in  Ascaris,  1 1 

division  of  nucleus  in  Ascaris,  12,  16 

blastodermic  layers,  18 

blastopore,  theory  of,  23 

pro- amnion  of,  35 
Bile  ducts,  112 

Birdsell  on  sympathetic  nerves,  81 
Bischoff  on  inversion  of  blastodermic  layers,  23 
Bladder,  urinary,  122 

Blastoderm  {fiKa(TT6s,  a  bud,  5f'p(Lia,  skin),  for- 
mation of,  17,  I S 

three  layers  of,  1 7 

gastrula  condition  of,  21 
stage  typical,  22 

bilaminar,  21 

layers,  inversion  of,  23 

historically  considered,  23,  24 

characters  of  layers  of,  24 

table  of  development  from  layers  of,  25 

four  layers  of,  26 

separation  of  embrj^o  from,  34 
Blastodermic  vesicle,  definition  of,  17 

growth  in  the  cat  of,  18 

invagination  of,  22,  23,  24 

as  yolk-sac,  35 

first  attachment  of  to  uterus,  53 
Blastopore  (ySAao-ros,  a  bud,    Tr6pos,  a  passage), 
discussion  of  in  meroblastic  ova,  22,  23 

diverticula,  near  to  in  Sagitta,  26 

connection  of  anus  with,  108 
Blochmann  on  extrusion  of  one  polar  globule  by 

parthenogenetic  ova,  14 
Blood,  origin  from  parablastic  cells,  21,  25 

corpuscles,  40 
Blood  islands  of  Pander,  39 
Blood  vessels,  formation  of,  40 
Body,  segmentation  of,  2 
Body-cavity.     See  Ccelom 
Bonnet  on  middle  blastodermic  layer,  21 

on  connection  of  uotochord  with  buccal  epi- 
blast,  68 
proto vertebral  cavity,  160 
Born  on  the  lachrymal  canal,  89 


Born — continued. 

cephalic  clefts,  102,  n. 

thyroid,  iii 

septum  spurium,  141,  n. 

auricular  canal,  142,  144 

ventricular  septum,  142,  n. 

foramen  ovale,  valve  of,  144 

pulmonary  veins,  144 
Boveri  on  polar  gloljules  in  Ascaris,  9 
Bowman,  anterior  and  posterior  homogeneous 

lamellte  of,  88 
Brain,  development  of,  61 

flexures  of,  63 

fissures  and  convolutions  of,  71 
Branchial  sense-organs  of  Beard,  76 
Bronchi,  109 
Brown,  H.   H.,  on  extrusion  of  part  of  nucleus 

of  seminal  cell,  12 
Budge  on  Ijonphatic  system,  158 


C^ciTM  (blind),  107 

Calamus  scriptorius  (writing  pen),  62 

Callender  on  4th  visceral  arch,  103 

Canalis  re-unieus,  91 

Capsule  of  lens,  87 

whence  derived,  87 
Capsule,  cartilaginous  nasal,  166 

periotic,  166 
Carnoy  on  polar  globules  in  Ascaris,  9 
Cat,  blastodermic  vesicle  of,  18 
Cauda  equina,  60 

Centra  [nii/rpuv,  a  point),  of  vertebrte,  2 
Centrolecithal    {.tevrpov,    centre,    XijKvdos,    oil- 
bottle),  ova  in  arthropods,  8 
Cerebellar  peduncles,  66 
Cerebellum,  62 

formation  of,  66 

amj^gdahe  of,  66 
Cerebral  vesicles,  37 

table  of  development  from,  61 

fifth,  or  bulbar,  63 

fourth  or  cerebellar,  66 

third,  mid-brain,  67  ' 

second,  67 
first,  68 
Chick,  pro-amnion  in,  34 

rudiments  of  cranial  nerve  ganglia  in,  75 
Choanse  {xiayos,  a  pit  for  melting  metal  in),  97, 

100 
Chorda  dorsalis,  32.     See  jSTotochoed. 
Chorda  tympani,  79 
Chordae  tendineee,  142 
Chorion  {xopwv,  skin),  definition,  42  and  n. 

junction  of  allantois  with,  44,  46 
Choroid  coat,  87,  88 
Choroid  plexuses,  62,  67,  70 
Choroidal  fissure,  85 
Chromatin,  filaments  of,  12,  16 
Chromosomes  {xpoiiixa,  colour,  craj^a,  body),  12 
Ciliary  body,  88 
Circulation,  fcetal  peculiarities  of,  155 

course  of,    155 

changes  of  at  birth,  157 
Cloaca,  108,  122,  123,  127 
Cochlea  (/coxAtay,   a  snail  with  a  spii'al  shell), 
canal  of,  90 

ganglion  of,  78 

in  birds,  92 

modiolus  of,  93 

bone  of,  93 

spiral  lamina  of,  93 


INDEX   OF   PART   I. 


Ill 


Ccelenterates  {ko7\os,  hollow,  ?fT«poj',  intestine), 

23 
Cceloni  (koTAos,  hollow),  layers  ot,  2,  23 

tj'pical  origin  from  invagination,  24,  26 

formation  of,  36,  99 

connection  of  niesoblastic  somites  with,  37, 

160 
formation  of  pleurte  from,  no 
of  perioardium  from,  135  I 
serous  cavities  from,  159 
Coelom-invaginations,  22 
connection   of,  with  notochordal  invagina- 
tion, 34 
Colohoma  iridis  (»co\i5j3uj,ui,  a  part   taken  away 

in  mutilaiiou),  S5 
Columns  of  cord,  antei'ior  white,  first  rudiment 

of,  59 

posterior  white,  59,  75,  7S,  n. 

structure  of,  60 
Commissure,  anterior  of  cord,  60 

i^'ri'V,  67 

posterior,  67 
Commissures  of  cerebral  hemispheres,  71 
Congenital  fissures  of  neck,  103 
Connective  tissue,  origin  from  parablastic  cells, 

21,  25 
Cornea  (horny),  substantia  propria  of,  87 

epithelium  of,  87 
Cornu  (horn),  anterior,  first  rudiment  of,  59 
Cornu  Ammonis(from  resemblance  to  the  liorus 

of  the  statue  of  Zeus-Ammon),  72 
Corpora  quadrigemina  (four-fold  bodies),  62.  67 

striata,  62,  69,  73 
Corpus  albicans  (white  body),  71 
Corjius  callosum  (the  thick  body),  71 

peduncle  of,  79 
Corti,  rods  of,  93 

Coste,  discoverj'  of  germinal  vesicle  by,  8,  n. 
Cotyledons  of  placenta  ((coruA??,  anything  hollow), 

51 
Cowper,  glands  of,  12S 
Craniata,  27 
Cranium,  165 

Crura  cerebri  {criis,  leg),  62 
Crusta  (crust  or  rind),  67 
Cryptorchismus  {Kpv:rr6s,  hidden,  upxis,  testicle), 

127 
Cunningham,  D.  F. ,  on  sulci  of  brain,  72,  n. 
Cutis  vera,  formation  of  from  outer  muscle  plate, 

161' 
Cuvier,  ducts  of,  13S,  139,  141,  151,  152.  153 

vestiges  of,  154 
Cyclostomata  {kvk\os,  a  circle,   a-rdfj-a,  mouth), 

parablastic  elements  in  blastoderm  of,  27 
heart  in,  136 
Cystic  duct  (kuo-tis,  the  bladder),  112 


De^idua  [dcddere,  to  fall  oif),  44 
structm'e  of,  46 
reflexa,  46 
serotina,  46 
vera,  46 

function  of  glands  of,  47 
thickness  of  vera,  48 
stratum  spongiosum  of,  49 
stratum  compactum  of,  49 
atrophy  of  glands  of,  49 
placental,  51 

giant  cells  in  serotina,  54 
separation  of  at  birth,  55 


Decidual  cells  of  Friedlander,  49 
Definition  of  anatomical  terms,  4,  5 

mesial, 

lateral, 

frontal, 

sagittal  {safjilta,  an  arrow), 

coronal  (corona,  a  crown), 

dorsal  (dorsum,  a  liack), 

vuntral  {venter,  belly), 

neural  (vtvpov,  a  cord), 

visceral, 

cephalic  (KecpdAi),  head), 

caudal  (cauda,  tail), 

axial, 
Deiters,  sustentacular  cells  of,  93 
Delamination,  process  of  in  blastoderm,  21 

non-occurrence  in  invertebrata,  22 

Lankester  on,  24 
Descemet,  membrane  of,  88 
Descriptive  terms,  4 
Deutoplasm,  7,  8 
Diujiluagm    [oid,    tiirough,    (ppdyfia,    a    fence,) 

159 
Directive  corpuscles..  9.     See  Polak  GLObULES. 
Discus  proligerus,  albumen  deposited  on  ovum 

by,  7 
Dohrn  on  pituitary  body,  68 
Dollinger,  researches  of  Pander  under,  23 
Ductus  arteriosus,  146,  150,    155 
lingnalis,  no 
thyreoglossus,  no 
thyroideus,  no 
venosus,  152.  155 
Duodenal  loop,  104 
Dursy  on  contiuuitj'  of  piimitive  with  neural 

groove,  32 
Duval  on  epiblastic  connection  of  blastodermic 
vesicle  to  uterine  wall,  53 


Ear,  a  specialised  branchial  sense-organ,  76 

development  of,  89 

accessory  parts,  93 
Echiuoderms  yix^^o^,  urchin,    5ep,ua,  skin),  im- 
pregnation in,  II,  12 
Ecker  on  the  Sylvian  fissure,  73 
Ectoderm  {sktos,  outside,  Sep,aa,  skin),  primitive, 
18,  19,  24 

invagination  of,  22,  23 
Egg  tubes,  124 

Eiasmobranchs  (riKdafj.a,  lamina,  Ppayx'a,  gills), 
coelom  invagination  in,  22,  n. 

primitive  groove  in,  23 

neural  crest  in,  73 

spinal  nerves,  anterior  roots  in,  74 

origin  of  ciliary  ganglion  in,  Si 

auditory  vesicle  in,   90 

supra-pericardial  bodies  in,  in 

liver  in,  112 

Wolflian  vesicles  in,  118,  n. 

suprarenal  capsules  in,  121 

lluUerian  duct  in,  122 

heart  in,  136 

muscle  plate,  outer,  in,  161 

visceral  arches  in,  161 

head  cavities  in,  161 

limbs  in,  163,  n. 
EmbiTo,    human,    segmentation    where     most 

marked,  2 
Embryo,  separation  of,  from  blasto'lerm,  34 
Embryology  (e]uj8(>i;ov,  a  thing  newlv  born.Ao-vo?, 
word),  definition.  I 


iV 


INDEX    OF    PAE.T    I. 


Embryonic  area,  i8 

Endocardium  {hSov,  within,  nap^ia,  heart),  136 

Endolymphatic  canal,  90 

Engelmaun  on  placenta,  48,  n. 

Enteric  canal  (evrepov,  intestine),  22 
groove,  36 

Entoderm  (ivr6s,  inside,  Sepfia,  skin),  primitive, 
17,  18,  19,  24 
blastopore,  relation  to,  22,  23 
perforation  of  by  primitive  groove,  22 

Entomeres,  mass  of  {iyT6s,  within,  /J-epos,  part), 

23  ,  ,      . 

Epencephalon  [iiri,  over,  iyKe(pa,\os,  brain),  66 
Epiblast  (eVi,  over,  ISAaards,  germ),  21 

homolog}^  with  Eauber's  layer,  22,  n. 

character  of  cells,  24 

formation  of  medullary  folds  by,  32 

formation  of  ganglia  by,  73,  75 

sensory  epiblast,  76 

formation  of  lens  from,  83 
Epiglottis  {eni,    upon,    yAcarTis,   mouth  of  the 

windpipe),  103 
Epiphvsis  cerebri  {iwKpvw,  to  gi'ow  upon,  cere- 
brum, brain),  6"] 
Epoophoron  (eTri,  u])on,  w6v,  egg,  (p^pa,  to  carry) 

ofWaldeyer,  120 
Eustachian  tube,  93 

valve,  141,  155 
Ewetsky  on  bifurcation  of  lachrymal  canal,  89 
External  amniotic  fold,  43,  n. 
Eye,  development  of,  S3 

retina,  86 

hyaloid  membrane  of,  87 

corneo-sclerotic  coat  of,  87 

choroid  coat  of,  87,  SS 

iris  of,  87,  88 

anterior  chamber  of,  SS 
Eyelids,  89 


Fallopian  tube,  7 

fimbriae  of,  1 1 

development  of,  117,  124 
False  amnion,  42,  43,  n. 
Falx  cerebri  (a  sickle),  70 
Fertilization,  11 

Filum  terminale  (terminal  thread),  60 
Fissure  of  Sylvius,  72,  73 

of  Eolando,  73 
riemming  on  Wolffian  duct,  118 
Fol  on  polar  globules  in  echinoderms,  9 
Foramen  caecum  of  Morgagni (blind  opening),  102, 

no 
Foramen  of  Monro,  63,  68 

ovale,  144 

closure  of,  155,  157 
Fore  brain,  38 

Fore-gut,  first  formation  of,  34 
Fornix  (an  arch),  71 
Foster   on   nomenclature   of    the   blastodermic 

layers,  23 
Fraser  on  inversion  of  blastodermic  layers,  23 

airditory  ossicles,  168 
Fretum  Halleri  (a  strait  or  channel),  141 
Friedlander,  decidual  cells  of,  49 

on  giant  cells  in  decidua  serotina,  54 
Froriep  on  branchial  sense-organs  in  mammals, 
76 

vertebral  column,  162 
Furcula,  the  (forked  prop),  102,  103 


Gadow  on  auditory  ossicles,  168 

Gall  bladder,  112 

Ganglia  {yayyMov,  a  swelling),  posterior-root,  74 

rudiments  of,  in  cranial  nerves,  75,  76 

Gasserian,  78,  81 

jugular,  78 

cochleae,  78 

spiral,  78 

geniculate,  78 

sympathetic,  81 

ciliary.  Si 

ophthalmicus  profundus  of  elasraobranchs, 
81 
Ganoids  {yavos,  bright,  elSoy,  form),  heart  in,  136 
Gartner,  duct  of,  120 
Gaskell  on  cranial  ganglia,  77 

nuclei  of  cranial  nerves,  77 

sympathetic  ganglia,  81 
Gastrula  {yavTrip,  belly),  21 

views  of  formation,  22,  23 
Gegenbaur  on  homodynamy,  4 

Jacobson's  gland,  97 

remains  of  notochord,  162 

auditory  ossicles,  168 
Generative  organs,  male,  117,  120,  125 

external,  127 

female,  124 

table  of,  130 
Genital  cord,  123 

ridge,  124 
Germ  cell,  12 
Germinal  cells,  58 
Germinal  epithelium,  117 

connection  with  Wolffian  tubules,  120 

formation  of  ovary  from,  124 
of  testicle  from,  125 
Germinal  spot,  8 
Germinal  vesicle,  structure  of,  8 
Giant  cells  of  placenta,  54 
Gill  slits,  103 
Giraldes,  organ  of,  120 
Girdle,  thoracic  and  pelvic,  164 
Globular  processes  of  His,  95 
Glomeruli  ((/ZomMS,  ball  of  thread),  119,  122 
Golowiue  on  ganglion  rudiments,  75 

special  sense-organ  rudiments,  76 
Gotte  on  Wolffian  duct,  11 8 
Graaf,  de,  on  pineal  e}"e,  68 
Graafian  follicle,  6 

union  of  cells  of,  with  ovum,  7 

changes  prior  to  escape  of  ovum,  9 

formation  of,  124,  125 
Gradenigo  on  the  stapes,  168 
Gruber  on  a  case  of  venous  abnormality,  155 
Gubernaculum  testis  (a  helm),  126 

changes  of  in  descent  of  testicle,  127 
Guinea-pig,  formation  of  amnion  in,  43 

allantois  in,  46 
Cyrus  subcallosus  of  Zuckerkandl,  81 


Hadden,  on  origin  of  segmental  duct,  118 
Haeckel,  on  gastrula  stage  and  gastrulisation, 

22,  23,  24 
Haller,  vasa  aberrautia  of,  120 
Hare  lip,  97 

Hassal,  concentric  corpuscles  of,  112 
Head,  first  appearance  of,  34 

muscles  of,  161 

evidences  of  .segmentation  of,  161 

skeleton  of,  166 

vi'seeral  skeleton  of,  166 


INDEX    OF    PART    I. 


Heape,  on  blastodermic  layers  in  the  mole,  i8 

rudimentary  blastopore  in  the  mole,  22 
Heart,  formation  of,  38,  136 

bej^inning  of  beat,  39 

endothelial  tube  of,  134 

folding  and  division  of,  137,  141,  142 

saccus  reuniens,  139 

septum  spuriuui  of,  141 

valves  of,  141,  142 

fretum  Halleri  of,  14I 

septum  inferius  of,  14I 

auricnlar  canal  of,  142 

septum  intermedium  of,  142 

chordoa  tendinece  of,  142 

foramen  ovule  of,  144 

limbus  Vieussenii  of,  144 

muscle  of,  145 

columme  curiRfe  of,  146 

foetal,  peculiaiities  of,  146 
Hedgehog,  placental  sinuses  in,  52,  n. 

epiblastic  attachment  to  uterus  in,  53 
Henle,  loops  of,  122 
Hensen,  canalis  re-uniens  of,  91 

on  Wolffian  duct,  118 
heart,  bilateral,  134 
Hermaphroditism  ['Ep,urjs,  '  AcppoSiTT),  a  male  and 

female  god),  12 
Hertwig,    0.    and    E. ,    on   polar  globules    in 
echinoderms,  9 

coelom-invaginations  of,  22 

coelom,  typical  development  of,  24 

mesodermic  laj'ers  of,  26,  27,  n. 

mesenchyme  of,  26 
Hind  brain,  38 
Hind  gut,  ibrmation  of,  36 

connection  with  allantois,  46 

with  Wolffian  duct,  118 
His,  "\V. ,  abdominal  stalk  of,  46 

on  accessory  thyroid  bodies,  1 10 

allantois,  loi,  n. 

arterial  arches,  147 

ascending  roots  in  medulla,  66 

auricular  canal,  142 

axis  cylinders,  growtli  of,  75 

blood-vessels,  151 

Broca's  area,  79 

cephalic  clefts,  102,  n. 

endothelial  tube  of  heart,  136 

on  epiblastic  origin  of  ganglia  and  posterior 
roots,  57 

germinal  cells  of,  58 

on  geniculate  ganglion,  79 

globular  processes  of,  95 

on  heart,  bilateral,  134 

isthmus  of,  67 

on  laminae  of  cord,  60 

lens  capsule,  87 

mechanical  theory  of  development,  24 

mesoblast,  origin  of  in  part  from  thickened 
rim  of  blastoderm,  23 

nerve  fibres,  somatic  and  splanchnic,  77 

olfactory  epiblast,  98 

nerve  fibres,  origin  of,  77,  81 

parablast,  theory  of,  25,  26 

on  pinna,  95 

porta  restibuli  of,  157 

on  pulmonary  veins,  144 

researches  of,  6,  n. 

saccus  reuniens  of,  139 

septiim  atriorum,  144 
inferius  of,  141 
between  stomodaeum  and  fore-gut,  99 


His — amtinued. 

septum,  transverse  of,  159 

sinus  arcuatus,  102 

on  spinal  nerves,  roots  of,  74 

spongioblasts  of,  58 

sulcus  terminalis,  102 

thyroid,  no 

on  tuberculum  impar,  102 

visceral  arches,  102 
Histologj'  [icrrov,   a  web;  K6yos,   word),  defini- 
tion, I 
Hofmann,  gastrulation  in  elasmobranchs,  22,  n. 
Holoblaslic  ova  (8aos,  whole  ;  /3Aost«{j,  a  germ), 
definition  of,  8 

invagination  of,  22 
Homodynamy  (<5m<5s,  the  same  ;  SwSjUis,  power),  4 
Homogeny  {.bixos,  the  same  ;  yivos,  descent),  4 
Homologj'  (6/u(Js,the  same ;  Kdyos,  word), definition 
of,  4 

serial  homology,  4 

examples  of,  4 
Hubrecht,  on  epiblastic  connection  of  blasto- 
dermic vesicle  with  uterus,  53 
Huxley,  on  two  layers  of  ccelenterates,  23 

auditory  ossicles,  16S 
Hymen,  the  (the  god  of  maniage),  128 
Hyoid,  the  (T  el5os,  the  form  of  the  Greek  letter 

T),  166,  169 
Hypoblast  (vTt6,  under  ;  0\a(rT6s,  genu),  21 

characters  of  cells  of,  24 

formation  of  notochord  from,  32 

allantois  from,  44,  46 
Hypophysis  cerebri  {inro(pva},to  gi-ow  from  below), 

68,  100.     And  see  Pituitapa'  Body. 
Hj'pospadias  {vtt6,  under  ;  crndw,  to  tear),  129 


Impregnation,  ii 
Incisor  foramen,  97 
Incus  (an  anvil),  167,  168 
InfundLibulum,  the  (a  funneD,  61,  67,  68 

of  lung,  no 
Intermediate    cell    mass,     the    formation     of 
Wolffian  duct  from,  115 

of  pprmanent  kidney  from,  122 
Intermaxillary  process,  95 
Intervertebral  disc,  162 
Intervillous  space,  51 

question  of  blood  in,  53 
Intestines,  104 

large,  106 
Invagination  in  holoblastic  ova,  22 

aperture  of,  22 

resemblance  to  blastopore,  22 
Inversion  of  blastodermic  layers,  23 
Iris,  86,  87,  88 

Ischikawa  on  segmentation  of  ovum  before  fer- 
tilization, 14 
Island  of  Eeil,  70.  73 
Isthmus  faucium.  100 
of  His,  67 


JaCobsox's  organ,  95,  97 

KAEYOKiifESis  {Kapvov,  a  kernel ;  Klvr](ns,  move- 
ment) of  germinal  cells,  58 

Karyoplasm  (Kapvov,  a  kernel ;  irXaaua,  a  mould) 
of  germinal  vesicle,  8 

Kessler,  on  lens  capsule,  87 

substantia  propria  of  cornea,  87,  n. 


VI 


INDEX   OF   PART   I. 


Kidney,  head  or  fore,  115,  117 

mid,  116 

hind  or  permanent,  117,  122 
KoUiker  on  Eaiiber's  layer,  18 

intermediate  layer  of  blastoderm,  20 

placenta,  53 

lens  capsule,  87 

the  anterior  chamber,  88 

cephalic  clefts,  102,  n. 

air  cells  of  lung,  1 10 

thymus,  iii 

heart,  134 

endothelial  tube  of  heart,  136 

mesocardium  laterale  of,  136 

cartilage  of  vertebice,  162 
Kollmann,  parablastic  theory  of  His,  adherence 

to,  26 
Kowalevsky,  researches  on  Sagitta  and  Amphi- 

osus,  24,  26 
Kiindrat  on  placenta,  48,  u. 
Kupffer,  gastrulation,  view  of,  22 

His'  parablastic  theory,  adherence  to,  26 


Labia  majora  (greater  lips),  128 
Labyrinth,  the,  the  recess  of,  90 

perilymphatic  spaces  of,  93 
Lachrymal  canals  and  ducts,  89,  97 
fissure,  89 
gland,  89 
Laminae    of   cord,    alar,    basal    (dorso-lateral) 

(ventro-lateral),  60,  63,  65,  66,  77 
Langhans  on  placental  sinuses,  53 
Lankester,  on  homogeny,  4 

blastopore,  22 

primary  layers  of  blastoderm  produced  by 
delamination,  24 
Larynx,  no 

Lemurs  [lemur,  ghost),  nasal  gland  in,  98 
Lens,  83,  86 

vesicle  of,  87 

transitional  zone  of,  87 

capsule  of,  87 
Leopold  on  placenta,  48,  u. 

giant  cells  in  decidua  serotina,  54 
Leydig  on  special  sense  organs  of  gill  clefts,  76 
Lieberkiihn  on  blastodermic  layers  in  mole,  18 

on  lens  capsule,  87 

zonule  of  Zinn,  87 
Ligamentum  arteriosum,  150 
Limbs,  4,  163 

segments  of,  164 

rotation  of,  165 

bones  of,  165 
Limbus  (a  band)  Vieussenii,  144,  157 
Literature,  recent,  of  the  OAiim,  14 

blastoderm,  27 

embryo,  formation  of,  41 

general  subject  of  embryonic  formation  and 
development,  41 

decidua,  55 

nervous  system,  development  of,  81 

sense  organs,  98 

alimentary  canal  and  glands,  lungs,  113 

m'inary  and  generative  organs,  132 

blood  system,  158 

seroiis  cavities,  muscles  and  skeleton,  169 
Liver,  106,  112 

weight  of,  113 

formation  of  veins  of,  151,  152 
Lock  wood,  on  protovertebral  cavity,  160 


Lungs,  development  of,  109 
Luschka,  on  urachus,  122 

notochord,  remains  of,  162 
LjTnpihatic  system,  158 


Mackay  on  arterial  arches  of  the  bird,  150 
Macula  germinativa  (germinal  spot),  8 
Malleus  (a  hammer),  167,  168 
Malpighian  corpuscles  of  spleen,  108 

of  Wolffian  body,  120 

of  permanent  kidney,  122 
Marshal],  vestigial  fold  of,  155 

Milnes,  on  head  cavities,  161 
Martin  on  Wolfiian  duct,  118 
Masius  on  epiblastic  connection  of  blastodermio 

vesicle  with  uterus,  53 
Mechanical  theory  of  development,  24 
Meckel,  J.  F.,  23 
Meckel's  cartilage,  166,  167,  168 
Medulla  oblongata,  62 

formation  of,  63 
Medullary  folds,  fonnation  of,  30,  32 

groove.     See  ISTeukal  groove, 
Membrana  limitans  of  retina,  externa,  86 

interna,  83,  86 

nictitans,  89 

reuniens,  superior  of  Eemak,  32,  162,  165 

tectoria,  93 

tympani,  93 
Membranes  of  cord,  development  of,  60 
Meroblastic  ova  (juepoy,  a  part,  ^Aaaros,  a  germ), 
definition  of,  8 

gastrular  stage  in,  22 

Haeckel,  view  concerning  blastopore  in,  23 

mesenchyme  in,  27 
Mesenchjone  {jxiaos,  middle,  X'^Mo^j  juice)    26, 

27 
Mesencephalon  {/xtaos,  middle,  eyKe(pa\os,  brain, 
67 

connection  with  optic  stalks,  85 
Mesentery,  the  {ixfaos,  middle,  evrepov,  intestine), 

104,  107 
Mesoblast,  formation  of,  20,  21 

origin,  varieties  of,  22 

His'  view  of  origin,  23 

character  of  cells,  24 

connection  with  mesenchj-me,  26 

paraxial  and  lateral,  32 

cleavage  of,  36 

formation  of  vessels  in,  39 

growth  of  with  allantois,  44 
Mesoblastic  somites,  37,  159,  162 

cavity  in,  37,  160 
Mesocardium,  anterius,  posterius,  lateralis,  136 
Mesocolon  [koXov,  great  intestine),  106 
Mesogastiium  {yaaTrip.  belly),  106,  107 
Mesonephros  (vecppos,  kidney),  116 
ilesorchinm  (opxis,  testicle),  126 
Mesorectum,  106 
Mesovarium,  126 

Metameres  (/uera,  following,  fj-spos,  a  i^art),  4 
Metanephros   {iJ-iTo.,    behind,    pe^pos,    kidney), 

117 
Metazoa  {iJ-ird,  after,  C'^ov,  animal),  18,  26 
Metencephalon  {fniTo.,  behind,  iyK€cpa\os,  brain), 

63 

Meuron,  de,  on  obliteration  of  human  cesopha- 
gus,  104,  n. 
thjrmus.  III 
Micropyle  [jxiKpos,  small,  -nvKrt,  opening),  11 
Mid  brain,  38 


INDEX    OF    PART    I. 


Vll 


M  ihalkovics  on  stonioiloeum  in  rabbit,  99 
snpra-rcnul  capsules,  120 
Mulleiiau  ducts,  123 
ovary,  124,  n. 
Minot,  polar  f;lobules,  theory  of,  12,  14 
on  placenta,  4S  n. 

chorionic   epithelium,   bounding   placental 
sinuses,  52,  n. 
Mitsukuri  on  supra-renals,  122 
Jlodiolus,  the  (nave  of  a  wheel),  93 
Mole,  primitive  ectoderm  in,  iS 

blastopore  in,  22 
Monotremes  (/xovos,  single,  rprjixa,  a  hole),  6 
amount  of  yolk  of,  8 
cloaca  in,  loS 
Jlorbus  cceruleus  (the  blue  disease),  158 
Jlorgagni,  hydatids  of,  124 
Jlorpholog}'  (juop<j)7J,  form,  \6yos,  word),  deter- 
mination of,  2 
Mouth,  formation  of,  99 

non-correspondence  of  primitive  with  per- 
manent, 100 
Mulberry  mass,  1 6 

Miiller,    E.,    on  muscular  tissue   of  primitive 
aorlre,  151 
Johannes,  [Miillerian  duct  named  after, 

117 
W.,  sense  epithelium  of,  86 
Miilleriau  duct,  117,  122,  123,  124,  127 

invaginations,  115,  u. 
Miillerian  tibres,  86 
Muscle  plate,  160 
inner,  160 
outer,  161 
Muscles,  159 
Myelospougium     {(ive\6s,     marrow,      0-770770$, 

sponge),  58 
ilyotomes  {fivos,  a  muscle,  re^vco,  I  cut),  4 


Kasal  processes,  95 

laminfe,  95 

septum,  95 
Ifaso-palatine  canal,  97 
IJerves,  iifth  ascending,  root  of,  66,  78 

vagus  and  glosso-pharvngeal,  ascending  root 
of,  66,  78 

iifth  sensory  fibres  of,  66,  78 
motor  fibres  of,  66 

sixth,  seventh,  66 

thii-d,  fourth,  67 

spinal,  formation  of,  73 

roots  of,  74 

axis,  cylinders  of,  75 

traces  of  ganglia  in  cranial,  77 

cranial.  75 

auditory,  78,  89,  93 

sympathetic,  81 

nuclei  of  origin  of  cranial,  77 

inferior  laiyngeal,  shifting  of,  149 
Iserve  fibres,  origin  from  neuroblasts,  58,  59 

origin  of  somatic  and  splanchnic,  77 

oltactory.  Si 
Kervous  system,  general  view  of  development,  57 
Iseural  canal,  32 

parietes  of,  structure  of,  57 

crest,  73,  75 

groove,  formation  and  closure  of,  30,  32 

formation  of  cerebral  vesicles  from,  37 
Keurenteric   canal   {fevpov,    nerve,    ivTepov,  in- 
testine), 22 

piercing  of  notochord  by,  32 


Neuroblasts  {vivpov,  nerve,  ^\aaj6s,  germ),  58, 

59,  74,  9S  .  .       , 

Neuroglia  {vtvpov,  nerve,  y\la,  glue),  origin  of, 

60 
Nose,  a  specialised  branchial  sense  organ,  76 

formation  of,  95 
Notochord  (i/aiToy,  the  back,  x^P^Vt  ^  string  of 
gut),  2 

formation  of,  32 

an  emltryonic  structure,  34 

structure  of,  34 

connection  w  ith  buccal  epiblast,  68 

formation  of  vcrtebrai  round,  162 

connection  with  cranium,  165 
Notochordal  canal,  34 
Xuck,  canal  of,  127 
Nuclei  of  origin  of  cranial  nerves,  77 
Nucleolus  of  ovum,  8 
Nucleoplasm  of  germ  and  sperm  cells,  1 2 
Nucleus  (a  kernel)  of  ovum,  changes  in,  6,  9 

chromatin  filaments  of,  12 
Nymphffi,  128 


QiSOPHAOL'S   (o'itraj  =  (pfpci),   tO   cariT,    (payuv,    to 

eat),  103,  104,  n. 
Olfactorv  {oJjacio,  to  smell)  area,  95 

bulb.  Si 

epithelium,  98 

glomeruli,  81 

pit,  95 

lobes,  62,  71 

junction  of  ganglionic  rudiment  with,  77 

formation  of,  79 
Olivary  tubercle,  65 
Omental  sac,  106 
Omentum,  great,  106,  loS 

gastro-hepatic,  or  small,  106,  113 
Omphalomeseraic  vein  {dfj.(pa\6s,  navel,  fj.e<Tapaioi; 

mesenter3\     See  Vitelline  A'eix. 
Onodi  on  sympathetic  nerves,  81 
Optic  chiasma  {x'^aana,  two  lines  placed  cross- 
wise), 85 

cup,  83 

nerves,  origin  from  fore-brain,  38,  67,  79 

stalks,  formation  of,  85 

thalamus,  62,  67 

tracts,  67,  85 

vesicles,  primarj",  61,  67,  S3 
Organ,  definition,  i 
Organization,  plan  of,  2 
'  •  Organs  of  the  lateral  line, "  76 
Os  artieulare  (joint  bone),  168 
Otic  vesicle  (o5s,  wtikos,  ear,  of  the  ear),  89 
Ovary,  6,  124,  125 

round  ligament  of,  126 

change  of  position  of,  127 
Oviparous  {ovum,  egg,  pano,  to  produce)  verte- 

brata,  8 
Ovum  (egg),  alecitlial,  9 

attachment  of  to  uterus,  46 

centrolecithal,  8 

deutoplasm  of,  7 

development,  changes  prior  to,  6 

gastrula  of,  21 

germinal  vesicle  of,  8 

holobiastic,  8 

human,  exce])tion  to  rule  of  separation  from 
chorion,  46 

human,  villi  in  earliest  described,  55 


Till 


INDEX   OF   PART   I. 


0  vuin — continn  ed. 
fertilization  of,  1 1 
niammalian,  descent  of,  8 
maturation  of,  9 
mp.roblastic,  8 
permanent,  125 

polar  globules,  extrusion  fi'om,  9 
primordial,  124 
segmentation  of,  16 
size  of,  6 
striae  of,  6 
structure  of,  6 
yolk  of,  7 
zona  pellucida  or  zona  radiata  of,  6 


Palate,  the,  97 

cleft,  97 
Pancreas  {ttuu,  all,  Kpsas,  flesli),  loS,  113 
Pander,  researches  on  blastoderm,  23 

mechanical  theory  of,  24 

blood  islands  of,  39 
Papillary  nrascles,  142 
Parablastic  cells  [irapd,  beside,  ^KacxTos,  germ), 

21,  25,  27 
Parachordal    cartilages     {irapd,    beside,    x°P^^h 

string  ;  near  the  notochord),  166 
Parker,  on  formation  of  xiphoid,  163 

stapes,  168 
Parovarium  {irapd,  beside ;  ovarium,  ovary),  120 
Pars  ciliaris  retina,  86 
Parthenogenesis     {irapdevos,     virgin  ;     yei'eais, 

origin),  polar  globules  in,  14 
Parturition,  separation  of  membranes  in,  55 
Paterson  on  origin  of  sympathetic  system,  81 

limb  muscles,  163,  n. 
Penis,  128,  129 

Pericardium  {^ep'-,  about ;  KapBia,  heart),  135,  159 
Peritoneum  (Trepi,  about ;  reiVco,  to  stretch),  159 
Perivitelline  space  {vitelhis,  yolk),  9 
Peters  on  hyomandibula,  168 
Petromyzon  {irerpos,  a  stone  ;  fiv^w,  to  suck  ; 

lamprey),  pituitary  body  in,  68 
Pfliiger,  egg  tubes  of,  124 
Pharyngeal  septum,  100 
Pharynx,  loi 

Pliiltrum  {cpiATpov,  loveliness),  95 
Pineal  gland  {pinea,  a  fir-cone),  61,  67 

as  rudimentary  eye,  68 
Pinna,  the  (a  feather),  95 
Pituitary  body  (pituita,  phlegm),  61,  6j 

formation  ot,  68 

in  Petromj^zon,  68 

connection  with  stomodseum    100 
Placenta  (a  cake),  44 

formation  of,  49,  51 

basal  layer  of,  5 1 

subchorionic  membrane  of,  51 

loculi  or  cotyledons  of,  51 

structure  of  sinuses  of.  52 

intervillous  spaces  of,  51,  53 

■weight  and  size  of,  53 

microscopical  sti'ucture  of,  53,  54 
Placental  decidua,  5 1 

sinuses,  structure  of,  52 
Plane,  median,  4 

Plasmas  of  Weismann,  nuclear,  nutritive,  ger- 
minal, histogenetic,  14 
Pleurae  {vXevpa,  a  rib),  1 10,  159 
Pleuro-peritoneal  cavity.     See  Ccelom 


Plica  gubernatrix  (guiding  fold),  126- 

semilunaris,  89 
Polar  disc,  1 1 

globules,  6 

formation  of,  9,  12 
where  observed,  9 
of  Ascaris  msgalocephala,  9 
of  plants,  9 
Minot's  theory  of,  12 
"Weismaun's  theory  of,  14 
Pons  (bridge),  62 

formation  of,  66 
Porta  vestibuli  (gate  of  the  vestibule),  157 
Prse-hyoid  glands,  no 
Pregnancy,  changes  of  uterus  in,  48 
Primary  placental  circulation,  53 
Primitive  groove,  formation  of,  19 

as  extension  of  blastopore,  22 
as  homologue  of  part  of  entoderm, 

23 
Baltour's  view  of,  23 
connection    with   neural  groove, 

3°,  32  . 

streak,  formation  of,  19 

an    ectodermal    thickening    in- 
dented by  blastopore,  22 
origin  of  mesoblast  from,  22 
connection  with  notochord,  32 
velum  [velum,  a  curtain),  100 
Pristiurus,  heart  in,  136 
Proamnion,  35,  42,  n. 
Process,  nasal,  95 

fronto-nasal,  95,  loi 
maxillarj',  97,  loi 
globular,  95 
tympano-hyal,  16S 
styloid,  169 
Processus  vaginalis,  127 
Proctodseum  {irpuicTos,  the  anus),  108 
Pronephros  (tt/j^,  before;  ve<pp6s,  kidney),  115 
Pronucleus,  female,  male,  9,  11,  12 

chromatin  filaments,  in,  16 
Prosencephalon (irpos,  besides;  ey/ce'^aAos, brain), 

68 
Prostatic  vesicle,  124 
Protovertebrse  {irpSiTos,  first ;  vertebra,  I  turn),. 

37 

See  Mesoblastic  somites. 
Protovertebral  cavitj^,  37,  160 
Pulmonary  blood-vessels,  no 
Pupillary  membrane,  87,  88 
Purkinje,  germinal  vesicle  of,  8 
Pyramidal  tracts,  the  last  to  be  medullated,  60 
Pyramids,  anterior,  65 


QtlADEATE,  the,  168 


Rabbit,  polar  globules  observed  in,  9 
pro-amnion  in,  35 
stomodaeum  in,  99 
Ramon  y   Cajal,   on  posterior  roots  of  spinal 

nerves,  75 
Rathke,  diverticulum  of,  68 
on  hypophysis  cerebri,  100 
visceral  arches,  103 
arterial  arches,  147,  150 
trabeculje  cranii  of,  166 
Rauber,  layer  of,  17,  18,  22,  n.,  23 

on  the  mechanical   theory  of   developmeutj. 
24 


INDEX    OF    PART   I. 


IX 


Recessus  pulmonales   (recesses  ot    the  lungs), 

159 
Reichert  on  villi  in  earliest  ovum,  55 

mantle  of,  70 

on  incus  and  stapes,  168 
Reid,  John,  on  foetal  circulation,  157 
Rein  on  the  polar  globules  in  the  rabbit,  9 
Remak  on  bilaniination  of  middle  blastodermic 
layer,  23 

membrana  rcuniens  superior  of,  32,  162 
Renal  organs,  embryonic  ducts  of,  opening  into 

allantoic  pedicle,  46 
Rensou  on  extrusion  of  part  of  nncleas  of  seminal 

cell,  12 
Restiform  bodies,  65 
Retina  (nte,  a  net),  origin  from  fore-brain,  38 

development  of.  83,  86 

membranffi  limitantes  of,  S^,  86 

rods  and  cones  of,  84,  86 

hexagonal    pigmented    epithelium  of,    84, 
86 

central  blood-vessels  of,  S$,  87 

ganglionic  layer,  86 

sense  epithelium  of,  86 
Ketzius  on  the  zona  radiata,  7 
Rhomboidal  sinus,  60 
Ribs,  development  of,  163 
Rodents,   inversion  of  blastodermic   layers  in, 

23 

Paterson  on  sympathetic  in,  81 

nasal  septum  in,  95 
Rosenbuig  on  "Wolffian  duct,  118 
Round  ligament,  126 

Ruckeit    on   ccelom    invagination    in    elasmo- 
branchs,  22,  n. 

heart  in  Pristiurus,  136 


Sabatiee,    on    origin    of   subclavian    artery, 
150 

Eustachian  valve,  action  of,  157 
Saccule,  78,  90 
Saccus  reuniens,  139,  151 
Sagitta,  origin  of  mesoblast  in,  22 

Kowalevsky  on,  24,  26 

mesenchyme  in,  27 
Salensky  on  the  stapes,  16S 
Sauropsida  {aavpa,   a  lizard  ;    oi^is,  look),  hyo- 

maudibular  arch  in,  168 
Rcalie  (scala,  a  stair)  of  cochlea,  93 
Scarpa,  gangliform  swelling  of,  78 
Schenk,  on  sj-mpathetic  nerves,  81 
Schmidt,  on  Thebesian  valve,  141 
Schneiderian  membrane,  81 
Schultze,  M. ,  on  rods  and  cones,  86 
Sclerotic  {(TK\-r]p6s,  hard),  88 
Scrotum,  127,  128 
Sedgwick,  on  "Wolffian  body,  115,  n, 

Miillerian  duct,  123 
Segmental  tubes,  118 
Segmentation  of  ovum,  complete,  incomplete,  8 

main  factors  of,  9 

before  fertilization,  14 

after  fertilization,  16 
Selenka,  on  inversion  of  blastodermic  layers  in 
rat  and  mouse,  23 

epiblastic  connection  of  blastodermic  vesicle 
with  uterus,  53 
Semicircular  canals,  78,  91 
Seminal  cell,  12 


Septum  lucidiim  (transparent  partition), ventriclft 
of,  70 
spurinm,  141 
inferius,  141 
membranaceum,  142 
intermedium,  142,  144 
transverse,  the,  159 
Serous  cavities,  159 
Sexual  cells,  origin  of,  14 

conjugation  of,  Minot's  theory  of,  14 
Sinus  arcuatus  (arched  hollow),  102 
venosus,  112,  138,  151 
urogcnitalis,  123,  128 
Skull,  formation  of,  166 

chondrification  of,  166 
Somatopleure  {awfia,  boily  ;   irXivpa,  the  side), 

35 
formation  of,  36,  99 
origin  of  amnion  from,  42 
Somites  (<raj/ua,  body),  mesoblastic,  4 
Spec,  on  connection  of  mesoblastic  cleavage  with 
notochordal  canal,  34 
epiblastic  origin  of  "Wolffian  duct,  118 
Spencer,  Baldwin,  on  pineal  eye,  68 
Sperm  cell,  12 

Spennatozoon   (airepfia,    seed  ;     ^wou,    animal), 
accession  of,  to  ovum,  6,  9 
fertilization  by,  1 1 
fate  of  tail  of,  1 1 
maturation  of,  12 
Spinal  accessory,    origin   from    basal    lamina, 

60 
Spinal  cord,  development  of,  57 
outermost  layer  of,  58 
anterior  comu  of,  first  rudiment  of.  59 
columns  of,  59 
fissures  of,  59 
anterior  commissure  of,  60 
dorso-lateral  and  ventro-lateral  laminae  of 

60 
filum  teiminale  of,  60 
Cauda  eqiuna  of,  60 
growth  of,  pari  passu  with  vei-tebral  canal, 

60 
membranes  of,  60 
Splanchnopleure     {ffrr\dyx''oy,     the    inwards  ; 
irAevpd,  the  side),  35 
formation  of,  36,  99 
Spleen,  108 
Spongioblasts  {(xirSyyos,  sponge  ;  ^XaarSs,  £:erm), 

58 
origin  of  neuroglia  from,  61 
Stapes  (a  stirnip),  168 
Sternum,  163 
Stomach,  104 
Stomodaeum  {arSna,  mouth),  68,  99 

connection  with  nose,  95 
Strahl,  on  Wolffian  duct,  1 1 8 
Stratum  compactum,  49 

spongiosum,  49,  55 
Subchorionic  membrane  of  Turner,  51 
Sulci  of  brain,  primitive,  72 
principal,  73 
Sulcus,  anterior  limiting  and  lateral  limiting, 

34,  35. 
terminalis,  102 
Supra-hyoid  glands,  no 

-pericardial  bodies,  ill 
Suprarenal  capsules,  120 
Suspensory  ligament,  113 
Symmetry  of  form,  4 
bilateral,  4 


X 


INDEX    OF    PAE.T    I. 


Sympathetic  ganglia,  8 1 

connection  of,  with   supra-renal   capsules, 

122 


Teeth,  109 

Tegmentum  (a  cover),  67 

Teleosteans    {reXeos,    complete,    oaTiov,    bone), 

Wolffian  duct  in,  118 
Telolecithal  ova    {reXos,  end,  and  \7]kv9os,  oil- 
bottle),  8,  27,  30 
Terminal  sinus,  40 
Testicle,  117,  125 

descent  of,  126,  127 
Testis.     See  Testicle. 
Textures,  general,  1 

Thalamencephalon  {ddXafios,  chamber,  4yK4(paKos, 
brain),  61,  67 

connection  with  optic  stalks,  85 
Thebesian  valve,  141 

Thomsen  on  ganglionic  traces  in  third  nerve,  77 
Thomson,  Allen,  on  primitive  aortse,  146 
Th3'mus  {6x101,  to  offer  as  a  sacrifice).  III 
Th3'roid  (Svpeo's,  a  shield),  102 

formation  of,  no 

pyramid  of,  no 
Tiedemann  on  rhomboidal  sinus,  60 
Tissues,  general,  i 
Toldt  on  mesentery  of  pancreas,  113 
Tongue,  100 

formation  of,  102 

foramen  ceecum  of,  102 

papillse  of,  102 

sulcus  terminalis  of,  102 
Trabeculse  cranii  [trahs,  a  beam,  diminutive  of), 

166 
Trachea  (rpaxus,  rough),  no 
Trigonum  olfactorium  {Tpl-y&vos,  triangular),  79 
Trophoblast     {rpocp-n,     nourishment,     fi\a<Tr6s, 

germ),  of  Hubrecht,  53 
Truncus  arteriosus,  151 
Tuberculum  impar  (unp)aired  tubercle),  102 
Turner  on  placenta,  48,  n. 

subchorionic  membrane  of,  51 
Type,  vertebrate,  main  features  of,  2 


Umbilical  cord,  36,  43,  104,  122,  151 

Umbilical  duct,  36 

Umbilical  vesicle,  36 

Umbilicus  (the  navel),  43 

Urachus  (ovpov,  urine  ;  ex'^)  to  hold),  122 

Ureter  [ovpeoo,  to  make  water),  122 

Urethra,  122,  128 

Uriniferous  tubules,  119 

Uterus,  mode  of  attachment  of  ovum  to,  46 

changes  in  pregnancy,  48 — 55 

regeneration  of  after  parturition,  55 

formation  of,  124 

bifid  stage  of,  124 

OS  and  cervix  of,  124 
Uterus  masculinus,  124 
Utricle  {utricidus,  a  little  womb),  91 
Uvea  {uva,  a  cluster  of  grapes),  84,  85,  86,  < 


Vagixa  (a  sheath),  124 
Valve  of  Vieussens,  66 
Vasa  aberrantia  of  Haller, 
Vascular  area,  39,  151 
lamina,  151 


Veins,  azj'^gos,  154 

cardinal,  151,  152,  153 
connection  of,  with  heart,  138,  139 
iliac,  transverse,  153 
inferior  cava,  152,  153 
innominate,  154 
intercostal,  su]}erior,  154 
jugular,  transverse,  154 
jugular,  primitive,  151,  152,  153 
primitive,  formation  of,  39 
portal  and  hepatic,  151,  152 
pulmonary,  144 
subclavian,  154 
superior  cava,  151,  154 
umbilical,  138,  151,  152,  155 
vitelline,  39,  40,  112,  138,  151,  152 
Velum  palati  (curtain  of  the  palate),  100 
Vena  ascendens,  152 
Vense,   advehentes   and  revehentes,    112,   151, 

152 
Venous  annulus  of  duodenum,  151,  152 
Ventricles,  lateral,  62 
third,  62,  67 
fourth,  62,  66 
fifth,  70 
Vererbungstheorie  of  Weismann,  14 
Vermiform  appendix,  107 
Vertebra  {verto,  I  turn),  2 

permanent,  162 
Vertebral  column,  development,  2,  162 
bodies,  2 
segments,  4 
Vertebrate  type,  2 

Vesicles  of  the  cerebral  hemispheres,  61 
Vestibule,  78 

Villi  {villus,  shaggy  hair)  of  chorion,  43 
vascularisation  of,  44,  52 
connection  with  glands  of  decidua,  47 
zone  of,  55 
Visceral  arches,  formation  of  ossicles  of  ear  from, 

93 

first,  or  mandibular,  loi,  102 

cephalic,  102 

second  or  hj'oid,  102 

third  or  thyro-hyoid,  103 

palato-pharyngeal,  103 

post-  and  i^ree-buccal,  103 

blood  supply  of,  103 

cartilaginous  bars  of,  103,  166 
Visceral  clefts,  103 

first  formation  of  middle  ear  from,  93 

cephalic,  loi 

hyoniandibular,  103 

fourth,  formation  of  thyroid  from,  no,  ni 
Vitelline  arteries,  40  ' 

circulation,  40 

duct,  46,  104,  151 

membrane,  8 

veins,  39,  40,  112,  138,  151,  152 
Vitellus,  structure  of,  7.     See  Yolk. 
Vitreous  humour  {vitrum,  glass),  8^,  85,  87 

"WAG^'EK,  macula  germinativa  of,  8 
"Waldeyer  on  His'  parablastic  thcorj^,  26 

placental  sinuses,  52 

epoophoron  of,  120 

germinal  epithelium  of,  124 
Weismann,  theory  of,  14 
Weldon  on  supra-renal  capsules,  120 
Wijhe,  v.,  on  Wolffian  vesicles,  118,  n. 

head  cavities,  161 

typical  somite  cavity,  161,  n. 


INDEX    OF    PART    I. 


XI 


WolfiF,  C.  F.,  first  suggestion  of  laminsB  in  blasto- 
derm by,  23 

duct  of,  115 
"Wolffian  body,  115,  n.,  116,  117,  llS,  122 

atrophic  changes  of,  120 

connection  with  testis,  120,  126 

veins  of,  151 
Wolffian  duct,  115,  Il6,  117,  iiS 

remnant  of,  in  parovarium,  120 

as  duct  of  Gartner,  120 

male  generative  oi'gans,  120 

pemianent  kidney,  122 

connection  with  Jliillerian  duct,  123 


Wolffian  vesicles,  118.  iiS,  n. 

Yolk,  structure  of,  7 
sac,  3";,  99.  104 
vitelline  circulation  round,  40 

Zacharias,  polar  globules  in  Ascaris,  9 
Zimmermann  on  arterial  arches,  151 
Zona  pellucida  (transparent  zone),  6,  7 

penetration  of,  by  spermatozoon,  1 1 
Zonule  of  Zinn,  87 
Zuckerkaudl,  gyrus  subcallosus  of,  81 


.JjOVM^J 


.i.LbL 


END   OF   PART    L 


m     - 


^^i 


J\P^, 


BRACEURT,    ACyKW,    &    CO.    LD.,    PRINTERS..   LONDOS  AKD  TOKBRITCE 


A    LIST    OF    WORKS    ON 

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A     TREATISE     ON     GOUT     AND     RHEUMATIC     GOUT 

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HALFORD.     THE   LIFE   OF    SIR   HENRY  HALFORD,   "Bart., 

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to  George  III.,  George  IV.,  William  IV.,  and  to  Her  Majesty  Queen  Victoria. 
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HtkLUBURTOH.— WORKS    by    W.   D.    HALLIBURTON,    M.D., 

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A     TEXT-BOOK     OF     CHEMICAL     PHYSIOLOGY     AND 

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LANG.  THE  METHODICAL  EXAMINATION  OF  THE 
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LIVEING.      HANDBOOK     ON    DISEASES    OF    THE     SKIN. 

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LUFF.      TEXT -BOOK     OF     FORENSIC     MEDICINE    AND 

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and  Children.  By  Various  Writers.  Edited  by  RICHARD  QUAIN,  Bart., 
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GEORGE  DANCER  THANE,  Professor  of  Anatomy  in  University  College, 
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WAKLEY.     THE  LIFE  AND  TIMES  OF  THOMAS  WAKLEY, 

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AN    INTRODUCTION    TO    HUMAN    PHYSIOLOGY.     Third 
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EXERCISES  IN  PRACTICAL  PHYSIOLOGY, 

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